you'd better believe it
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.