American English, colloquial, by 1863, from drawling pronunciation of yes.
Entries linking to yeah
yes adv.
Old English gise, gese "so be it!," probably from gea, ge "so" (see yea) + si "be it!," 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. As a noun from 1712. Yes-man is first recorded 1912, American English.