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1.
n. The act of opening one's backdoor, that is, anal sex
"Hey, you know I wanna be your backdoor key, its about time I opened it up"

John: "Did you hear about Matt?"
Chris: "No what happened?"
John: "While he was fucking Shirley she gave him the key to her backdoor"
by Mitch Tight and Bright March 07, 2011