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1.
Similar to blood brothers, when you have sex with a girl, you become box brothers with every guy she's slept with before.
Doug: Man, I'm sorry, I banged Sheryl last night.
Mark: Sweet dude, we're box bros now!

or

Sheldon: What a night, I ended up going home with Kristie.
Steve: Shitty, you just became box brothers with like 20 dudes.
by Crabzilla November 25, 2009