bet one's bottom dollar

To be totally certain that something will happen or prove true (so much so that one would hypothetically risk one's last dollar betting on it). I lost my umbrella, so you can bet your bottom dollar on it raining tomorrow! I bet my bottom dollar that Kevin will be late tonight—he's never on time! Well, we know the rest of the family has gotten to town, so if Mom's calling you repeatedly, I'd bet my bottom dollar they're making her miserable.

One may be very certain of something. Bottom here means last, or the bottom of a pile of gambling chips. An American colloquialism of the mid-nineteenth century, the phrase became a cliché in the mid-twentieth century.