When a dude is trying to get with a girl or currently talking to her. If his friend tries to talk to her as well, despite the fact of knowing his friend is entertaining the girl...AND she falls for it....that girl has been "backdoored."
(Setting: Girl is invited to a small house party by a teammate)
Aj: Hey yall this my friend Tasha
Teammates: Hey...what's up...(wassup nods)

An hour later
Random Teammate: Hey Tasha, my name is Chris. Can I get your number I want to get to know you a little more.
Tasha: Sure, it's 305-432-3443

Following morning
Tasha's friend 1: You know Tasha is talking to AJ's friend Chris now?
Tasha's friend 2: Aw man...they totally back door her
by JKardashian March 29, 2010
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