You 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 better believe it!

verb
See You had better believe it!
See also: believe, better
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • You had better believe it!
  • you('d) 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
  • pass the baton
References in periodicals archive
His big-band compositions displayed an adventurous approach with complex voicings and harmonies were spotlighted on a series of critically acclaimed recordings for the Pacific Jazz label, including such albums as "You Better Believe It!,'' "Moment of Truth'' and "Portrait.''
Jazz musician Gerald Wilson dies
She might have made us feel rather sheepish this year, but we'll be wiser next year, you better believe it!
Jenny Asper the jokester! This "April Fools" prank left Albawaba blushing
I'm going rock climbing in Iraqi Kurdistan.Aa You better believe it!" I typed before I grabbed my cameras and ran out to the waiting car.
Off the beaten path
Organisms are in charge, you better believe it! The grandness and the range of scale, from gross patterns in the rock record to global geochemical cycles to cell metabolism, are captivating.
Biomineralization
You better believe it! For example, remember the good old days when we would show and sell and show and tell?
Leasing Specialists New Skills for a New Millenium
Like much work done by women, the preparation, editing and publishing of the journal was juggled between four PhDs, several research projects, heavy teaching loads, a book and the placement of over a thousand students (we counted them so you better believe it!).
Editorial