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!

Inf. a way of emphasizing a previous statement. Bill: Man, you're the best goalie this team has ever had! Tom: You better believe it! Bill: This food is so bad. It will probably stunt my growth. Tom: You'd better believe it!
See also: believe, better
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

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/you’d better beˈlieve it!

(spoken) used to tell somebody that something is definitely true: ‘He’s not a bad player, is he?’ ‘You’d better believe it!’
See also: believe, better
Farlex Partner Idioms Dictionary

You better believe it!

verb
See You had 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
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

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/you'd better believe it!
  • 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
  • baton