Green’s Dictionary of Slang

(orig. US) yes; also used as vb (see cit. 1930).

1891		Harper’s Mag. Nov. 970/1: He gently and peacefully murmured, ‘Yep.’ [DA].
1899		H. Garland Boy Life on the Prairie 8: ‘Is that the name of your horse?’ ‘Yup.’ [Ibid.] 389: ‘Had breakfast?’ ‘Yup.’.
1901		Boy’s Own Paper 5 Jan. 209: ‘Yep,’ replied the Yankee boy, Leo, ‘yep.’.
1906		Century mag. (N.Y.) Jan. 410/2: ‘Will you go — if I swear?’ ‘Yup,’ said Pinchas, airing his American [DA].
1915		Wodehouse Psmith Journalist (1993) 261: Mr Dawson said Yup, there sure was.
1920		‘Sapper’ Bulldog Drummond 14: ‘Yep,’ said the American shortly, ‘These two gentlemen agree with me.’.
1922		S. Lewis Babbitt (1974) 115: Yump, not hardly up to snuff.
1923		E. Wyatt Invisible Gods 78: ‘Yup,’ replied Hancock indifferently.
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.
1930		J.T. Farrell ‘Sunday’ in Short Stories (1937) 243: The old codger yepped agreement.
1937		F. Gruber ‘Death on Eagle’s Crag’ in Goulart (1967) 183: ‘Yep,’ said Quade.
1945		(con. 1944) R. Westerby Mad in Pursuit 262: ‘Still the Boy Scout, aren’t you, Tony?’ ‘Yep. Still.’.
1959		A. Zugsmith Beat Generation 97: Yep. I got the company to transfer me.
1964		L. Hansberry Sign in Sidney Brustein’s Window in Three Negro Plays (1969) I ii: Are you for real? [...] Yup.
1978		R. Caron Go-Boy! 187: Yup, sure enough he’s my prisoner.
1978		P. Theroux Picture Palace 249: ‘Yep,’ he said.
1987		B. Chatwin Songlines 128: ‘Yup,’ the woman nodded.
1989		C. Hiaasen Skin Tight 162: You’re sure . . . that this man is a murder suspect?’ ‘Yep.’.
1990		S. Morgan Homeboy 141: Yup [...] Twelve in a box.
1996		K. Lette Mad Cows 4: Yep. God was laughing when he made women.
2000		Guardian G2 9 Feb. 3: But now he’s back? Yup.
2006		G. Iles Turning Angel 278: Yep, good as new.