Meaning

noun an affirmative

adverb not only so, but

originFrom Middle English ye, ȝea, ya, ȝa, from Old English ġēa, iā (“yea, yes”), from Proto-West Germanic *jā. The modern pronunciation shows an irregular development of Early Modern English /ɛː/ to /eɪ/ in the standard language, probably from association with the antonym nay.

Etymology from Wiktionary, CC BY-SA 3.0