you bet your ass
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]