you/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

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