you can bet your bottom dollar (on something)

You can be certain that something will happen or be the case (as if 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! Oh, you can bet your bottom dollar that Kevin will be late tonight—he's never on time!

bet one's bottom dollar, one can

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.