When you do something amazing that would normally take more than 4-5 people.
In Modern Warfare 2:

Guy 1: "Hey you just killed the entire enemy team in Ground War!"

Guy 2: "I know, I'm a one man gangbang!"
by DoctorGoodWood May 28, 2010
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When one man starts a gangbang and no one taps him out because he's just that good.
Amanda: Hey look at Billy go

Sarah: Yea, he's a regular one man gangbang

Or

Amanda: hey Sarah how's your ass feeling today?

Sarah: Surprisingly not bad after having Billy, The One Man Gangbang. He's gentle!
by TeamNoSleep420 December 11, 2017
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