Origin and history of yeah

Middle English yis, from Old English gise, gyse, gese "so be it!," probably from gea, ge "so" (see yea) + si "be it!," reconstructed to be from Proto-Germanic *sijai-, from PIE *si-, optative stem of root *es- "to be."

Originally stronger than simple yea. Used in Shakespeare mainly as an answer to negative questions. Yes, yes, indicating impatience, anxiety, enthusiasm is attested by mid-15c.

As a noun from 1712, "an utterance of 'yes,'" hence "assent, affirmative reply." As a verb, "assent," by 1820; as "flatter by agreement," by 1921.