Green’s Dictionary of Slang

(orig. US) yes.

1899		Ade Fables in Sl. (1902) 77: ‘Home agin, Jethro, be ye?’ asked the Parent. ‘Yeh,’ replied the Educated Youth.
1926		L. Pound ‘Popular Variants of “Yes”’ in AS II:3 132: All the listed below were known to many persons among the hundreds questioned. [...] yip, yep, yap, yop, yahp, yup, yurp, yis, yuss, yays, yass, yahss, yazz, yahzz, ye-us, ye-yuss, yeh, ye-ah, yessir (yes sir).
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 173: ‘Yeah,’ the needle-faced guy said. [Ibid.] 225: Yeah, don’t fool yourself!
1947		B. Schulberg Harder They Fall (1971) 47: That was the most combative Danny ever got, that ‘yeah.’.
1965		C. Brown Manchild in the Promised Land (1969) 64: Yeah, that’s what I forgot to say to Mama.
1979		E. Torres After Hours 33: Yeah I sound like last legs.
1988		Eble Campus Sl. Oct. 11: Yeah, bubba – affirmative response.
1999		K. Sampson Powder 173: ‘Is that it?’ [...] ‘Well, like, yeah.’.
2000		Guardian Guide 1–6 Jan. 11: Yeah! Boy, I want to be healthy now.