knock oneself out

(idiom) go ahead!

Example Sentences:

Sarah told Tommy to knock himself out after Tommy asked her if he could stay at her house for the weekend.

A: I’m having a party tonight.
B: Is it alright if I come?
A: Sure, knock yourself out!
B: Thanks!

Sister: Do you mind if I have a bite of your ice cream?
Sister: Knock yourself out! It’s delicious!