What does "Knock yourself out." mean?

Question

What does  "Knock yourself out."  mean?

Answer

I think of it as "Go right ahead" or "Go for it" as ian mentioned.

It's basically just a way of saying "Sure, of course you can!"

"Can I ____?"  "knock yourself out!" (sure you can! please do! be my guest!...)

"If you would like to/any ______________, then knock yourself out" (meaning don't hesitate to do it/take it.)

You can also say "go nuts", which is kind of interesting because that's also an idiom for "go crazy"

Answer

knock yourself out = go nuts = go ahead and do it, enjoy :P

Answer

It means, do as much as you like, or have as much as you like!

"I want some of your food, can I have it?"   
"Knock yourself out!"  (Have as much as you like! Go for it!)

Answer

It means: feel free to do whatever you like.