you'd 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
Farlex Dictionary of Idioms.

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]
See also: believe, better
The American Heritage® Dictionary of Idioms by Christine Ammer.

you'd better believe it

You can be absolutely sure. An Americanism of the mid-nineteenth century, it appeared in print in 1856. The Toronto Globe and Mail used it in 1968: “You’d better believe it . . . We’ve got ’em.” For a similar affirmation, see take it from me.
See also: believe, better
The Dictionary of Clichés by Christine Ammer
See also:
  • You better believe it!
  • You had better believe it!
  • you('d) better believe it
  • you/you'd better believe it!
  • bend over, here it comes again
  • BOHICA
  • I can't believe it/that/this
  • I can't believe!
  • hand over the baton
  • pass the baton
References in periodicals archive
"If they say they're going to do something, you'd better believe it," she says.
Death without justice: after their teen son committed suicide in a panic, Pennsylvania parents sued the police who'd threatened to out him. The resulting court case is a landmark for gay and lesbian privacy rights. (Court)
JOHN COLTRANE Ascension (Impulse!): When BBC's Saturday evening jazz documentary slot devotes a whole series of programmes to one album, you'd better believe it's a major work.
JAZZ CD OF THE WEEK; JAZZ CDS