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!
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.
Thanks for the explanation, Delmobile.