What does bet mean? You're using Dictionary to look up something you don't know? Bet! Bet is a slang term of affirmation, agreement, or approval along the lines of Cool!  or I'm down!  It can also suggest doubt or disbelief: Yeah, sure. 

: something that is laid, staked, or pledged typically between two parties on the outcome of a contest or a contingent issue : wager. often used figuratively in such phrases as all bets are off to stress the uncertainty of an outcome.