you (can) bet your (sweet) ass

rude slang You can be absolutely certain that something will happen. Sometimes used ironically. You bet your sweet ass I'm going to that concert—I've been saving up to buy a ticket for months now! Oh, you can bet your asses that Kevin will be late tonight—he's never on time! I can't find my umbrella, so of course you can bet your ass that it'll rain soon.

rude slang Absolutely. You bet your ass I'll be there on Friday!

Also, you bet or you can bet your bottom or bottom dollar or (sweet) life . You can be absolutely sure, as in You bet your ass I'll be there, or You can bet your sweet life that was Bill with another woman, or Are you coming?-You bet I am. All these phrases in effect mean that you can be so sure of something that you can wager your body or life or valuables on it; ass is considered vulgar, and bottom dollar means "last dollar." [Colloquial; second half of 1800s]