1. The last dollar one has available to spend. I put my bottom dollar on that bet. If I lose this one, that'll be the end of my Vegas run. Sorry, dude, I can't loan you anything—I spend my bottom dollar on that sandwich.

2. A certainty that something will happen or be true. (Used almost exclusively in the phrase "bet (one's) bottom dollar.") I lost my umbrella, so you can bet your bottom dollar that it will rain tomorrow! Oh, I'd bet my bottom dollar that Kevin will be late tonight—he's never on time!