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!"
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]
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.