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3.
n. when a man attempts anal sex without requesting verbal permission. Often involves either precursory acts such as foreplay (rimming, anal fingering) or preceding acts (spreading of ass checks, tapping head of penis on anus, pushing the tip of the penis onto the orifice without full insertion) to signal one's attempt at anal intercourse.
After doing it doggystyle with Jackie, I started knocking on the back door. She didn't object so I totally stuck it in her butt until she screamed.

Richard Johnson kept knocking on the backdoor last night in bed, but I told him to stop since I barely know him.
by calisp July 01, 2013
 
1.
Pressing the tip of one's erect penis against a woman's anus in the hope that she'll agree to anal sex. This technique is futile 99.753% of the time, but persists due to the collective male delusion that women secretly love to be invaded in this way.
Boyfriend: (knocking on the back door)

Girlfriend: How many times do I have to tell you??? Get that thing away from there!!!
by Carl Pinkerton May 16, 2009
 
2.
When a man feigns an attempt at anal penetration in order to startle his partner.
Woman: I told you I don't do that!

Man: Don't worry, I'm just knocking on the back door.
by wiggy_G November 20, 2009
 
4.
During anal sex, the person that is doing the penetrating and thrusting is the one that is doing the "knocking," and the anal opening is, obviously, referred to as the "back door."
When Eric and Amy are in bed, he's always knocking at the back door.
by Zeeky H. Bomb March 19, 2005