How did “acker” come to mean friend in West Country English?

I actually asked a few people from Somerset and they had never heard it so it's not a well-known word.

Its etymology seems to be very complex and perplexing. I searched quite a few etymology dictionaries but didn't find its origin.

As user121863 says:

The etymology may be simply unknown or lost in time.

I'm not an expert but I'm going to speculate.

It's probably derived from Old English gemæcca (or maca), which means companion, mate, friend, equal etc. (mentioned by a linguist on Reddit in his comment).

Make and match are derived from the same word gemæcca.

Quoting from Etymonline:

Make (n.): "match, mate, companion" (now archaic or dialectal), from Old English gemaca "mate, equal; one of a pair, comrade; consort, husband, wife," from Proto-Germanic *gamakon- (source also of Old Saxon gimaco, Old High German gimahho, Old Norse maki), related to Old English gemæcc "well-matched, suitable," macian "to make"). Meaning "manner in which something is made, form, shape, design, construction" is from c. 1300. Slang phrase on the make "intent on profit or advancement" is from 1869.

Match (n.2): "one of a pair, an equal." Middle English macche, from Old English mæcca "companion, mate, one of a pair, wife, husband, one suited to another, an equal," from gemæcca, from Proto-Germanic *gamakon "fitting well together" (source also of Old Saxon gimaco "fellow, equal," Old High German gimah "comfort, ease," Middle High German gemach "comfortable, quiet," German gemach "easy, leisurely"), from PIE root *mag- "to knead, fashion, fit."

Meaning "person or thing that exactly corresponds to another" is from c. 1400. Middle English sense of "matching adversary, person able to contend with another" (c. 1300) led to sporting meaning "contest," which is attested from 1540s. Meaning "a matrimonial compact" is from 1570s.

There's also makker in Dutch.

From Middle Dutch macker, probably from *gemacke (“companion”). More at make, match. [Wikitionary]

It's clear that gemæcca and makker are closely related.

There's also Macker in German which was borrowed from Dutch makker.

Etymology of macker: From German Low German Macker (“companion; guy”), first attested in 1771 and hence possibly borrowed from Dutch makker (“mate, fellow”) (1557). Further origin uncertain, but likely related to Old Saxon gimaco (“companion”), Old English maca, ġemaca (“companion, mate, wife, one suited to another, idem”), Old Norse maki (whence Swedish make (“husband”)), from Proto-Germanic *makkô, *gamakkô, *makô, *gamakô (“an equal; comrade”), from Proto-Indo-European *mag- (“to knead, work”). Compare also French mec, which is of uncertain origin, but probably from Dutch. [Wikitionary]

According to Wikipedia, macker is derived (or closely related) to Old English gemæcca.

So I'd guess English borrowed macker from German and it became widespread. Then later on it lost the m due to Apheresis and became acker with the same meaning (friend, companion).

(Mucker is also a common word that means friend in some parts of the UK, I guess it's also related but I don't have any information about that.)

(There seems to be a good explanation here but it's in Dutch and I don't know Dutch (translation might be incorrect)).

According to Macmillan Dictionary Blog:

Match itself is centuries older and has quite complex origins. It was derived partly from the Old English word gemæcca, which lost its ge- prefix through aphesis (apheresis). This is a process where the initial sound of a word is dropped, as in because → cos, it was → ’twas, and acute → cute.

Originally, match meant an equal, or a partner, a complement, one of a pair. Then gradually it developed a new sense, of a contest between two ‘matched’ parties or players. These are the kind of matches that take place at Wimbledon and other sporting events; they appear as sense 2 in Macmillan Dictionary. In US English the word game is more usual in this context.

'Acker' in the nineteenth century

Whether acker in the sense of "friend or mate" is Somerset-specific or not, the term probably arose within the past 100 years. William Holloway, A General Dictionary of Provincialisms (1840) has nothing for acker in the relevant sense, nor does James Halliwell, A Dictionary of Archaic and Provincial Words (1852) or Thomas Wright, Dictionary of Obsolete and Provincial English (1857) or James Jennings, The Dialect of the West of England, Particularly Somersetshire, second edition (1869) or Frederick Elsworthy, The West Somerset Word-Book (1886) or J. H. Blascke, A Few Steps to a Complete Dictionary of English Dialects (1890) or Joseph Wright, The English Dialect Dictionary (1898).

The absence of any visible nineteenth-century traces of acker as "friend or mate" puts a crimp in any theory that purports to derive it from an Old English predecessor.

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'Acker' as an Oxford -er coinage

It seems at least as likely that acker is an instance of a not altogether uncommon use of the suffix -er to form what Michael Quinion, Ologies and Isms: Word Beginnings and Endings (2002) calls "Colloquial or humorous nouns and adjectives":

-er³ Also -ers. Colloquial or humorous nouns and adjectives. {Probably an extended use of -ER¹ ["a common and productive suffix, with several senses"]}

Examples are footer (football), rugger (rugby), brekker (breakfast), and soccer (the last from an abbreviated and clipped form of Association Football). The style is to abbreviate (and often to distort—the root word, an then add -er. This was originally Rugby School slang, later adopted at Oxford University about 1875, then extended into general use. Most examples have since disappeared; only soccer has become standard English. A few are spelled -ers (Twickers for the rugby ground at Twickenham), though most with this ending are adjectives: bonkers and crackers (mad), preggers (pregnant), starkers (stark naked).

Eric Partridge, A Dictionary of Slang and Unconventional English, eighth edition (1984) actually lists an instance of "ackers" that seem to follow this pattern—although in this case the term doesn't carry the meaning "friend or mate":

ackers. Activity at physical exercises: Pangbourne Nautical College: since ca. 1950 (Peppitt.) [Origin:] The 'OXFORD/R[OYAL] N[AVY] -ER(S)'.

Two additional instances, both from Australia, show up in Jonathon Green, Chambers Slang Dictionary (2008):

acca n.¹ (also acker) {abbr. + pun on OCKER n. (4) [that is, "Australian English"]} {1970s+} (Aus.) 1 an academic, esp. one who trades on the proliferation of current, if ephemeral, intellectual fads. 2 quotidian, jargon-laden academic writing.

acca n.² (also ack, acker) {abbr.} {1980s} (Aus.) acne.

So acker[s] has emerged as a slang term derived from activity (in England), from academic (in Australia), and from acne (in Australia).

If acker in the sense of "friend or mate" followed this same pattern, what word might it have come from? The choices aren't great, but a few aren't beyond the pale: accompanist, accomplice, acolyte, acquaintance. Be that as it may, I have not found any authority that traces acker to any of these words.

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'Acker' in the wake of Mr. Acker Bilk

There is very little to go on in trying to trace the origin of acker in the sense of "friend or mate" before the emergence of "[Mr.] Acker Bilk and His Paramount Jazz Band," which began to receive frequent mentions in Google search results in 1960. A Billboard item from April 28, 1962, notes that part of Bilk's shtick was that he was something of a rube from jazz-inhospitable Somerset:

Next upon the British scene was Acker Bilk. Publicized as a rustic eccentric by Peter Leslie, his fame as a concert attraction preceded his disk success. This came in late 1960 and Bilk was in the British charts almost weekly last year. His biggest success is "Stranger on the Shore."

And an item in a 1959 issue of Jazz Music includes this note about a May 9, 1955, recording by "Master Acker Bilk's Paramount Jazz Band" with "Acker Bilk" on clarinet [combined snippets]:

Apparently, these four tracks were the first to be recorded by Acker Bilk under his own name and Esquire, the label that has done much to put more names on the road to fame saw it fit to release four exceptionally good numbers by this young man from Somerset. It is also fitting to note that here we have Master Acker Bilk (the custom later to call him Mister Acker Bilk) in all his glory.

The most noteworthy point here is that Bernard Stanley Bilk was evidently billing himself as "Acker Bilk" at least as early as 1955 (he was born in 1929). An unidentified article in Quadrant (1982) addresses the acker in "Acker Bilk" as part of its inquiry into the origin of the Australian slang term ocker [combined snippets]:

Several correspondents who knew ocker characters some decades ago observed that the description 'ocker' was used with little or nothing to do with a given or family name and simply means 'fellow' or 'mate' The term could well be one of many Australian words which came from British dialects and there is a body of evidence, some quite circumstantial, to support this. A glance at a few standard reference works yields a number of clues, any one of which might have been the trigger which gave us 'ocker'. In isolation they may not seem very convincing but a word does not have to be commonplace or have a particular modern meaning before it can take a place in the language. ...

Of considerable interest is an assertion by Acker Bilk, the British jazz musician, that 'acker' is a Somerset term for 'mate'. It is probably related to the above examples and, if so, its form would be a relatively recent development. The evidence in documentation is hardly definitive but we should be prepared to accept that this usage exists, at least in the English oral folk tradition. In support of this another informant from the same general area of England recalled a common childhood phrase 'good my acker', readily translatable into Australian as 'good on you mate'. Thus 'acker' appeals as quite a likely source and the form of the word approaches that of the Australian version. A retraction of the short open 'a' in some English speech or simple error in transmission would easily have made 'acker' sound like 'ocker' to Australian or other ears in some past time.

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Conclusions

I have not been able to find an earlier reference to acker meaning "friend or mate" in Somerset or West Country slang than the 1982 Quadrant article. Presumably, the term truly is a dialect word for "friend" in Somerset. Simon Elmes, Talking for Britain: A Journey Through the Voices of a Nation (2006) says as much:

acker a friend. Somerset usage; the jazz musician Acker Bilk, born in Pensford, Somerset, took his professional name from this dialect word

But the lack of any recorded mention of acker in this sense before the rise of Acker Bilk is striking—and somewhat puzzling. The slang term may be older than Mr. Bilk, but if so it led a remarkably stealthy existence prior to his emergence as a musical celebrity. In any event, I see no reason to suppose that acker meaning "friend or mate" goes back much further than the middle of the twentieth century.

In Ireland, mostly the North, people refer to a friend as 'Mucker' which comes from the Gaelic 'Mo Chara'- (mo kara) which means my friend. Acker might be a version of this brought to the west country with immigrant workers from Ireland.

Paul Flynn is more right here even than he realises. It's likely not from immigrant workers but part of a vernacular core of Gaelic language that includes luv (as, in the West Country, my luvver), just as where I live (Pennine South Yorkshire). Luv is clearly Gaelic: Scottish Gaelic leibh, a contraction of le sibh, a composite pronoun & preposition meaning ‘with you, together with you, on your side, in your favour, by you’ — le is the preposition ‘with, together with, in company with, on the same side with’, and denotes feeling; sibh is the personal pronoun, ‘ye, you’. [Gaelic bh is always pronounced ‘v’, of course.] It crops up everywhere and at any juncture, notably in ta luv ('thanks to yer'). This is Scottish Gaelic tapadh leibh, pronounced (capitalising the stressed syllables) ‘TAH-puh LEH-eev’, ‘thanks to you’ (formal). Ta luv is standard shortening over time to leave out the medial unstressed syllable.