Green’s Dictionary of Slang

(US) a general excl. of affirmation or agreement.

1981		M. Baker Nam (1982) 41: I was given to Fox Trot [company]. I said, ‘That’s a bet. I like that name.’.
1988		W.D. Myers Fallen Angels 286: ‘Peewee, we get the direction of the voices, then go the other way.’ ‘Bet!’.
1994		G. Pelecanos Shoedog 101: ‘Have a good night, man.’ Randolph raised his brow. ‘Bet’.
1995		G. Pelecanos Down by the River 17: ‘How about that kid, the one that got it two nights ago [...] . I guess that was a drug thing, too.’ ‘Bet it’ .
2000		W.D. Myers ‘Monkeyman’ in 145th Street 80: ‘How about tomorrow night, eleven o’clock [...] ?’ Monkeyman said. ‘Bet!’ the Tigros dude said.
2023		Sydney Morn. Herald 18 Nov. 🌐 Bet is another way of saying OK.