Welcome to English in a Minute. To knock someone out in the sport of boxing, you must hit the
other boxer really hard. So does knock yourself out mean you hit yourself?
Another invitation to the grand fan band jam.
What? I have been waiting to see that band for years, Dan. Here, take my tickets. Are you sure?
Knock yourself out. I know the drummer, so I hear that band all the time.
What? You know the drummer?
If someone says knock yourself out to you, they are telling you to go ahead and do something.
This informal expression shows you they are not doing anything.
This informal expression shows you they don't really care either way.