you'd better believe it

you'd better believe it idiom
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® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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