+0
Jane: Hey Ashley, do you mind lending me your laptop? My laptop just crashed for the fifth time this month.

Ashley: Sure. Knock yourself out.

What does 'knock yourself out' mean here?

Thanks in advance!
+0
This is an ironic use. "Knock yourself out" means to completely exhaust yourself doing something. (A "knockout" is a boxing term for a punch that renders the other fighter unconscious; we say "Smith knocked Jones out in the second round." At least I think we do; I'm not much of a boxing fan.)

"All I want to do is rest for the next couple of weeks. We had Jim's entire family in town for the Christmas holidays and I absolutely knocked myself out cooking big meals and driving them around playing tourist."

People sometimes use the expression in a slightly sarcastic way.

Child (in awed wonder at older teen's electronic game): Wow! That game is really cool! Do you think I could play it sometime?

Teen (handing game over): Sure, kid. Knock yourself out.

The general idea is: you're welcome to it; feel free to exhaust yourself using it.
Comments  
Thanks for the explanation, Delmobile.