I bet!

Từ điển Oxford Advanced Learner 8th

I/I'll bet! idiom
(informal)
1. used to show that you can understand what sb is feeling, describing, etc.
•‘I nearly died when he told me.’ ‘I bet!’
2. used to tell sb that you do not believe what they have just said
•‘I'm going to tell her what I think of her.’ ‘Yeah, I bet!’
Main entry: ↑bet_idiom