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1.
It's when a woman takes anal beads, shoves it up the man's ass, and proceeds to twist them around in a "door-knob-esque" fashion.

She then removes the anal beads from his ass, and smacks him repeatedly in the face with the dirty beads.

Hence, we have a dirty doorknob.
"I dirty doorknobbed that faggot good!"
by Shawn12345 August 09, 2008
 
2.
during sex, you lube up your fist, aim, and upper cut your partner in the ass almost hard enough to fist them. the partner will scream and grab the closest thing to them, mainly the doorknob out of the house.
tyler: dude, would you ever give kat a dirty doorknob?

stupid skinhead: yeah, and my brother in the same session.
by thunder shaft July 10, 2008