The strange pronunciations of "assume"

Brief answer

Asyoom is sometimes pronounced ASHOOM in some accents because there's a tendency to assimilate (coalesce) ‹S› with the following ‹Y› to a ‹SH› sound. The same thing happens in bless you when pronounced quickly. Another common example is whatcha (what + you).

Explanation

'Assimilation' is what causes all these changes. It's a process which makes nearby sounds more similar (opposite: dissimilation e.g. colonel pronounced kernel).

There are place assimilation, voicing assimilation and assimilation of manner.

• In assimilation of place, one of two adjacent sounds changes its place of articulation in order to make it more similar to the other sound, for example, ten pies is usually pronounced te[m]pies in fast (sometimes normal) speech (the place of articulation of the n changes).
• In voicing assimilation, one of the two adjacent sounds changes its voicing. For example, the B in absorb changes to a P when the suffix -tion is appended: absorption.
• In assimilation of manner, one sound changes its manner of articulation to become similar in manner to a neighbouring sound. For example, that side is sometimes pronounced [ðæs saɪd]; the plosive /t/ changes its manner of articulation and becomes a fricative.

Pronunciation of ASSUME as ASHOOM

It's an example of coalescent assimilation. In this type of assimilation, two adjacent sounds are merged/coalesced to form a new sound. The following sounds often merge and make new sounds:

• [s] + [j] → [ʃ]: Mission, assure, sexual etc
• [z] + [j] → [ʒ]: Vision, treasure, usual, azure etc
• [t] + [j] → [t͡ʃ]: bet you (betcha), what you (whatcha) etc
• [d] + [j] → [d͡ʒ]: did you (didja), would you etc

Assume is pronounced [əˈsjuːm] in most British and Australian accents. If you check dictionaries, you will find [əˈsjuːm] for British and [əˈsuːm] (without the /j/) for American pronunciation¹ (therefore it may not be common in American accents). However, some people—mostly British and Australians—assimilate the [s] with the following [j] and pronounce it something like [əˈʃuːm].

There are lots of other interesting examples—mostly historical—that illustrate the process. Some of them are:

• The prefix in the word impossible is in- but has been assimilated to an m because of the following /p/. N often assimilates to /m/ before /p m b/ etc
• Most people (including me) pronounce the and in 'bread and butter' and 'bits and bobs' as if it were an m in anticipation of the following sound
  • bread[əm]butter
  • bits[əm]bobs
• Bless you being pronounced with [ʃ]
• Tune is usually pronounced [t͡ʃuːn] (CHOON) in most varieties of British English
• Sure and sugar
• Some people pronounce handbag [ˈhæmbæɡ]: the first process involved in this case is simply the elision of the [d] and then the assimilation of n and b to m.
• Have to pronounced hafta

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_{/s/ is the 's' in sin
/j/ the 'y' sound in you
/ʃ/ is the 'sh' in ship.
/ʒ/ in genre
/t͡ʃ/ in church.
/d͡ʒ/ in judge.}

^{1: "In many accents of English, /sj/ at the start of a syllable has been simplified to /s/. This simplification has progressed further in North American English than in British English"}