1
general::
1. Make a great effort, as in I was knocking myself out to finish on time. This expression Also is put negatively, Don't knock yourself out, meaning ‘‘don't exert yourself; it's not worth that much effort." [c. 1930] 2. Enjoy yourself, have a good time, as in You're off to Europe? Knock yourself out. [Slang; mid-1900s] Both usages allude to knocking oneself unconscious (see KNOCK OUT). For a synonym see BREAK ONE'S ASS.
American Heritage Idioms