It's a slang for the term anal. Meaning you put the male genitalia into the female's rectum.
E.G for key to the backdoor:

Male : I'll give you the key to the back-door baby!
Female : I'm not that dirty thank you... I don't like anal.
by Husky123654 December 16, 2014
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To penetrate the anus of a human being or some other animal if you're into the whole bestiality thing.
Angie got pissed at Jimmy for using the backdoor without anal-eze
by Elvis's Sideburns May 20, 2015
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to use hummus as lube for anal sex
Todd : “Yeah man she’s a vegan, dunno if i should smash”

Jack : “She’s a vegan? She might enjoy a bit of hummus in her ass”

Todd : “Vegan Backdoor Entry? Totally Tubular!”
by axelotyll November 12, 2020
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When someone takes a massive escargot diarrhea shit into another person baggette favored starfish while holding a beret.
That escargot yesterday could really bring up our moral if your down with a European backdoor tonight. Or now
by Lookslikeabluewaffleday March 10, 2021
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A sex position involving a Netipot
Brian stayed up late, hoping for the delaware backdoor, but the pharmacy was already closed.
by Black Cloud 99 December 30, 2016
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The act of introducing Irish Whisky into the rectum & colon via the anus (Butt-Chugging)
“Do you wanna go back to my apartment and have an Irish Backdoor?”

“Do you guys know how to get to Ireland from here? Sure, take your pants off and I’ll give you The Irish Backdoor”
by FuccTheNiners March 08, 2020
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