Also, you better believe it. You may be assured, as in You walk ten miles every day?—You'd better believe it! This imperative is almost synonymous with you bet your ass. [Colloquial; mid-1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.