A western-Pennsylvania bastardization of the phrase "you ones", which
addresses more than one person. Since we "western PA people" do things like slur words
together, replace almost all vowel sounds with
a "schwa" sound (the upsidedown e in the dictionary), replace long "a" sounds with
long "e" sounds, and refuse to pronounce any "t" that isn't the first letter of a word, this became "you-ens", then
"youns", "yunz", and finally
"yinz". Someone must have put this particular
pronunciation and spelling
in print at some point, since it seems
to be the most accepted
way to say and write it.
The aforementioned characteristics of our accent are probably at least
partly due to the diverse
heritage of the population of this area, which
is mostly made up of Italian, Irish, Polish, Native American, and African
American people, NOT inbreeding, as someone in an earlier entry stated.
The word has become associated with
Pittsburgh in particular
, whose natives are commonly referred to as "yinzers".