It has always confused how my American relatives pronounce the name of their city (Plymouth Meeting) as something like 'Plymeth Meeting'.
For me, it seems that the natural way would be something that sounds more like saying the words Ply and Mouth separately.
What is the history of this pronunciation in particular? Can it be explained by any more general principles about the relationship between the pronunciation and the spelling of proper nouns and place names?
As Michael Broughton observes, the American city is named after the English town, which is pronounced the same way.
The first part isn't pronounced like "Ply" because it's not the same word and doesn't have the same historical origin. The second part does appear to have originally been the word "mouth", but because it is in an unstressed syllable, it appears the vowel sound has evolved differently in the place-names ending in "-mouth" and the independent word "mouth". James posted a link to a very relevant Wikipedia article, the Toponymy section of "History of Plymouth". According to Wikipedia:
The modern name has two parts: Plym and mouth. The element Plym is taken from the River Plym along which it traded with its parent settlement of Plympton, but its name (first recorded as Plymentun in c. 900) is considered to derive from the Old English word for 'plum tree', also ploumenn in Cornish, though the local civic association suggests an alternative derivation from the Celtic Pen-lyn-don ("fort at the head of a creek"). An alternative derivation is from Latin plumbum album 'British/white lead' -meaning tin - where plomm is also Cornish for lead.
By the early 13th century, the river was being called the Plym (Plyme, in 1238), as a back-formation from Plympton and Plymstock (first recorded as Plemestocha in 1086). The earliest records of the name Plymouth date from around this time (as Plymmue in 1230, Plimmuth in 1234).
It looks like in spelling, the double "mm" eventually was reduced to a single one, but this was not accompanied by a change in pronunciation.
The single "u" was also later replaced by "ou"; this may have occurred at a time when the vowel was pronounced like the vowel in the independent word "mouth," or it might have just occurred by analogy with the spelling. The reason the vowel is not currently pronounced the same as the vowel in "mouth" is likely due to the process of vowel reduction that applies to many unstressed syllables in English.
A fact that supports the idea that this is related to stress was brought up in the following comment by JHCL:
In Britain, there are a few such place names that rhyme with "South" as commonly spoken (for example Avonmouth, Lossiemouth, Cockermouth), but as far as I can tell they all seem to be where the "-mouth" suffix comes after an unstressed syllable. Most, as with Plymouth, Dartmouth and Bournemouth, are said with a schwa. (Although I've heard Tweedmouth pronounced both ways.)
Syllables in English words have a tendency to alternate between being stressed and unstressed, so it makes sense that reduction might occur in syllables directly following stressed syllables, but not in syllables directly following unstressed syllables.
In general, the spelling and pronunciation of English placenames are often fairly remote from each other (even more so than for ordinary words). Cobaltduck linked to a Wikipedia article with a list of unintuitively pronounced place names that are even "worse" in this regard.
We tend to put the stress on the first syllable in many English words, especially place names.[I.e.There are many 'mouths'; Which one? Oh, the one on the Plym!]So the 'mouth' becomes as short 'u' sound, as in 'but', or even 'e' as in 'fed'.The Italians find it hard to understand Brits struggling with Italian, because they tend to stress every syllable evenly. Try saying ManCHESter instead of MANchester or LivERpool instead of Liverpool and you get the point. As for different spellings people tended to write down what they heard and so there were many variations. Remember too many people were illiterate then. So Sir Walter Raleigh is meant to have had his name spelt 126 different ways. That take some effort, to get it so "wrong"!
The inhabitants of a town or city use the name of their town or city very often. No wonder that there is a tendency to find a weakened or shortened form that is easier and faster to pronounce.This is a general tendency that can be found in other languages, too.
