What's the origin of this expression recorded in Louisiana, 1867?

As JEL had noted in an earlier answer to this question, the expression "piss through one quill" appears in dual-language dictionaries going back at least to 1688 when it appeared in The greate French Dictionary, with the explanation that the expression means "agree perfectly together." The same expression also appears in dual-language dictionaries of English-Italian (1727), English-Danish (1754), English-Swedish (1757), and English-Dutch (1766). And in Dictionarium Latino-barbarum (1677), a dual-language English-Latin dictionary, it appears as an English idiom under the word quill, in the form "They both piss in a quill."

John Ray, A Collection of English Proverbs (1678) lists the expression under the heading "Proverbial Phrases and forms of Speech that are not Entire Sentences," without further comment, in this form:

To piss in the same quill.

John Farmer & William Henley, Slang and Its Analogues Past and Present, volume 5 (1902) argues that the expression "piss through a quill" means simply "to write." Farmer & Henley may have been influenced by Abram Palmer, Folk-Etymology: A Dictionary of Verbal Corruptions or Words Perverted in Form or Meaning, by False Derivation or Mistaken Etymology (1882) who cites two relevant examples of "piss through a quill" (or "pissed in a quill") cited by Farmer & Henley in its later treatment of the expression. Palmer asserts that the expression is of Irish English origin:

In Ireland there is a coarse phrase of the same origin [as "in the quill," meaning "by joint action"] by which persons who are great chums, or hail-fellows-well-met, are said "mingere in uno quill (= rivulo)," "They p—— in the same quill."

[Example:] He would have us believe that he and the Secretary p——d in a quill ; they were confederates in this No Fanatic plot.—North, Examen, p. 399 {Davies}.

Marvell has the phrase in a somewhat altered form:— "I'll have a council shall sit always still, / And give me license to do what I will ; / And two secretaries shall p—— {mingent} through a quill." Poems, p. 188 (Murray repr.).

[Third example:] Thou runn'st to meet thy self's pure streams behind thee, / Mazing the Meads whre thou dost turn and winde-thee, / Anon, like Cedron, through a straighter Quill, / Thou strainest out a little Brook or Rill. [—]J. Sylvester, Du Bartas, p. 433 (1621).

A 1633 edition of the translation of Du Bartas that Palmer cites provides additional context for the usage there, but I still find the usage in that instance baffling.

The instance by Andrew Marvell occurs in a poem titled "Royal Resolutions" and does seem to use the expression to mean "write in ink"—although the quality of the writing (and of the reasoning behind it) is suggested by the metaphor. Marvell died in 1678, so this is a fairly early instance of the exact phrase "piss through a quill."

North is Roger North, and Examen is Examen, Or An Enquiry Into the Credit and Veracity of a Pretended Complete Historian, published in 1740, but written sometime between 1711 and 1734, when North died.

It appears that the forms "piss in one quill" and "piss in a quill" are older than "piss through one [or a] quill." I suspect that the quill in question isn't a feather or spine, but an obsolete sense of the word, which The Compact Edition of the Oxford English Dictionary (1971) gives its own entry, as follows:

†Quill, sb., Obs. rare. {? a. OF. *quille = F. cueille gathering, harvest, sb. f. cueillir (OF. quillir, etc.) to gather, CULL v.} 1. = Coil, sb.3 ["A length of cable, rope, etc., when 'coiled' or gathered up into a number of concentric rings..."] [Example from 1588 omitted.] 2. In the (or a) quill: In a body; in combination or concert. To jump in quill, to act simultaneously or in harmony. [Examples from 1593, 1687, and 1690 omitted.]

This would explain how in the following instance, from "An Humble address to the most illustrious and high born James Francis Edward, present Prince of Wales" (1688), hundreds of people could be described as having "Piss'd all in one Quill":

In setting of thee up, they spoil'd their Plot, / And now their Names will ever Die and Rot: / And if thou Liv'st but ten Years more to come, / Thou may'st then hear there was a Pope of Rome, / And hundreds more, who Piss'd all in one Quill, / No Laws could bind them, only their own Will.

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The expression "through one quill" is not common in the book and newspaper databases that I consulted, but it does appear occasionally. For example, from "My Fust Gong," in the Goulburn [New South Wales] Herald and Chronicle (August 22, 1868):

That ar things in this life tu big tu be trifled with, thar ar times when a man brakes luce from hsself, when he sees speerits, whe he kan almost tuch the moon, and feels as tho he kud fill both hands with the stars ov heavin and almost sware he was a bank president.,? Thats what ailed me. But the korse ov tru luv never did run smoove (this is Shakespere's opinion too, i and he often think through one quill).

And from an untitled item in the Santa Cruz [California] Sentinel (December 9, 1909):

Under the head of "The vacancy on the Council," the [Santa Cruz Evening] News publishes on its editorial page what appeared editorially in the [Santa Cruz] Surf. When evening sheets blow through one quill how harmoniously a vacancy in the Council glides along. The ammunition that one manufactures two can fire.

The sense of the expression in these newspaper instances seems to be "with one voice" or "in complete accord"—essentially the same meaning it had in 1688 in The greate French dictionary.

The phrase, 'piss through one quill', appears in an English publication as early as 1579, in this variant:

...the Iewes, who doe agree so well in one with our deare mother the holie Churche of Rome, that they pisse both in one quill, and haue all studied in one schoole.

From George Gilpin's English translation of the Dutch satire Roman Beehive, by Philips of Marnix. Emphasis mine.

I have been unable to discover whether the translation was from Dutch into idiomatic English, or was a word-for-word translation of an existing Dutch idiom.

The earliest attestation of the phrase (and variants) in OED is from sometime (unknown) before 1641:

J. Smyth Prov. in Berkeley MSS (1885) III. 32 Things ne're goe ill where Jacke and Gill pisse in one quill.

In OED, the definition of the phrase is "to agree, to be of one accord; to have a close relationship."

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Incidentally, the 'we' in "we dam-d bitches ass" was likely the regional adjectival use, equivalent to 'our', observed by the OED:

C. adj.
regional (now chiefly Caribbean). As a possessive adjective: = OUR adj.