knock oneself out

idiom

Definition of knock oneself out 

1 : to cause oneself to become unconscious He hit his head against the table when he fell and knocked himself out.

2 informal : to make oneself very tired by doing work I knocked myself out to get the job done on time.

3 US used to tell someone to go ahead and do something "Do you mind if I use this exercise machine first?" "Knock yourself out."

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