What is the origin of the pronunciation of words like Leicester, Gloucester, Worcestershire? Presumably, the spelling predates the pronunciation but what is the history here? What language do the words originate from?
Is this just simple elision or is there a more specific, shared origin of these words and the process that changed them?
These are all quite old names with many syllables—an ideal place for ellipsis, elision, and contraction.
The -cester bit is from the Old English word ceaster, which in itself is borrowed from Latin castrum. It is thus cognate to castle. (The palatalisation of the initial /k/ to /tʃ/ is common in Old English and was more widespread in some dialects than in others, so at some point, there were probably two variants around: caster /kastər/ and ceaster /tʃastər/.)
In Old English, ceaster referred to a Roman town or settlement (in England). As tchrist notes in his comment, some other places that have this element in their names include Lancaster, Doncaster, and Manchester.
The first part of names that have -cester tend to be inherited place names of either Anglo-Saxon or (even earlier) British Celtic stock, often names of rivers or other local toponyms.
Going from Old English down through the centuries, the cities the first element of whose names ended in a consonant (like Lan-, Don-, Man-) usually kept a fuller version of the word ceaster, retaining the initial consonant. The ones whose first element ended in a vowel, on the other hand, ended up having the initial /tʃ/ (or /k/, as in Lancaster and Doncaster) in an intravocalic position, where it was quite likely to be weakened—something that happens often in place names, especially longer ones.
Thus, Leicester and Gloucester were once pronounced as they’re written, with a /tʃ/ (or perhaps /k/); but that consonant was weakened over time and eventually disappeared altogether, leaving the vowels free to contract into single monophthongs, too.
The cester/chester ending can be traced back to the Roman occupation: castra is Latin for camp, so these placenames indicate Roman army encampments (or more likely permanent garrisons, given the names' persistence).
The Grammarphobia blog gives a good run down of the evidence which traces these pronunciations through history; the scansion in Shakespeare, for example, suggests that the elision was in common use in his time. The article also speculates on why the pronunciation and spelling are so divergent: perhaps because the people writing down the placenames (the literate scribes of Domesday Book; the monarch's tax-collectors &c) weren't the ones living there, coupled with the usual spelling free-for-all that only started to be regularised with mass-printing.
Sometimes pronunciation can swing back into line with spelling; while Cirencester is now generally pronounced as spelled, it had been elided to cicester.
Similar elisions are common in other patterns of placenames (Kirkby:kirby; Kircudbright:Kurkoobree; Marylebone:marleybone/mairbun/m'bn) and so probably represent a universal trend towards elision in familiar and locally-shared placenames.
The relative lack of shortening in such names as Manchester, Lancaster, Chichester, Colchester looks to be caused by the middle consonant 'catching' the speaker and preventing further elision. The examples in the question, by contrast, don't have a strong middle consonant.
I would consider my answer personal observation due to a software project. I believe it is a general trend. It would be helpful, if people would offer corrections to my observation.
Human attention span seems to constrain a concept to be pronounced no longer than 3 syllables. This human tendency is due to the angst of wanting to say as much as possible within a given time.
I can say this about English and Hebrew, since these are the languages I am familiar with. Perhaps someone could verify that French, Chinese, Hausa, Japanese, etc succumb to the same constraint.
In standard British English, three syllable words are often contracted to two-syllables during pronunciation. If that is not possible or inconvenient, then words are normally contracted to 2.5 syllables.
In standard British English pronunciation, a word usually has 2 major syllables and 1 minor syllable = 2.5 syllables. Another frequent pattern is two major and two minor syllables = 2 + 0.5 + 0.5 = 3.
These are the most common patterns of pronunciation (M=major, m = minor syllable) for three syllable words.Frequently, pattern #2 is contracted to pattern #4.
For those rare words, or chemical names, a word is broken up into groups where the pronunciation of a group conforms to the above patterns.
There also is the principle of emphasis on the first syllable, so that a word is pronounced
m1-M2-M3
in order for the pronunciation of m1 to be sufficiently diminished to allow M2 to effectively be the first syllable. That is, at all times emphasize the pronunciation of the 1st syllable - but if that sounds silly, suppress the 1st syllable to allow the 2nd Syllable the prominence of a 1st syllable. (Beyond the scope of this question, I have developed a syllable valency rating which documents what syllables get to be suppressed - such that the lower the valency, the higher its "risk" of being suppressed.)
Examples:
Compare the contraction of Government to common Indian English
Government => M-m-M => M---M (GOU'ment)
Another phenomenon is the metathesis of multiple single syllable words into one word group to be pronounced as if they were a single word conforming to the three syllable patterns.
OTOH, there is the reverse-contraction phenomenon, which is rare in British English but of which Americans are excessively guilty. The extension of 2 syllable words to 2.5 or 3 syllables
So yes, in Shakespeare's First Folio, you'll see the spelling "Gloster"...
...but I just figured that if you organize the letters into syllables differently, then each word is indeed spelled just as it is pronounced.
Leice|ster
Glouce|ster
Worce|ster|shire
I might even speculate that "Manchester" is actually being pronounced Manche|ster, because if you try to say "Manche" followed by "ster", the reduced "e" automatically inserts itself... and generally, when I hear Brits pronounce the name, I hear that the e is indeed reduced. [EDIT: Likewise Chiche|ster.]
