You/you’d better believe it!
Definition 1 of 1
Pro Tip 1/3Strong Agreement
Use it to strongly agree with someone or support their claim, showing no doubt. 
A: 'This movie is a masterpiece.' B: 'You'd better believe it!'
Pro Tip 2/3Emphatic Assurance
It adds emphasis and confidence to your statement, making it more convincing. 
I aced the test, you'd better believe it!
Pro Tip 3/3Conviction Tone
Conveys a tone of conviction; you're telling someone to be as certain as you are. 
She's the best player on the team, you'd better believe it!