You had better believe it!

you('d) better believe it

Absolutely, without question; rest assured. A: "Man, he's the fastest runner this school has ever had!" B: "Yep, you'd better believe it!" A: "And is the movie really scary?" B: "You better believe it!"
See also: believe, better

You (had) better believe it!

exclam. It is true without question! Yes, this is the best, and you had better believe it!
See also: believe, better
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