From a variation of yea, itself from Middle English ya, ye, from Old English ġēa (“yes”), from Proto-Germanic *ja (“yes; thus, so”), from Proto-Indo-European *yē (“already”). Cognate with West Frisian ja, Dutch ja, German ja, Danish ja, Norwegian Bokmål ja, Norwegian Nynorsk ja and Swedish ja, Icelandic já. Interestingly, although perhaps merely coincidentally, modern "yeah" seems to possess nearly the same pronunciation as its Old English ancestor (Old English "ġēa" being /jæːɑ/, Modern English "yeah" being [jɛə̯] or [jæə̯]) and consequently has really no other words that rhyme with it in rhotic dialects. The reason behind its seeming lack of pronunciation shift is uncertain–although it is quite possible that it simply evolved from a dialectal form of yea which developed a centring diphthong–as other words that had similar pronunciations in Old English did shift in pronunciation. It is additionally possible that, due to its constant use (and its never being accepted as a formal term), its pronunciation has remained rigidly in place.
yeah (not comparable)
Note: Many languages have no equivalent of this word; in those cases, the translations given here are for yes.