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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]

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 .