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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?
Knock Yourself Out
Speaker A: Another invitation to the Grand Fan Band Jam.
Speaker B: What?! I have been waiting to see that band for years, Dan!
Speaker A: Here, take my tickets.
Speaker B: Are you sure?
Speaker A: Knock yourself out! I know the drummer. So, I hear that band all the time.
Speaker B: What?! You know the drummer?! Ahh!!!
If someone says “knock yourself out” to you, they are telling you to go ahead and do something. This informal expression shows you they don’t really care either way.
*This article has been edited and reprinted from VOA Learning English with permission from Voice of America (VOA) for use in English language materials.
English 
日本語