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It is literally a rubber band hanging on the doorknob, used to warn a roommate(usually in college dorms) that the other is currently having sex in that room.
If your ever having sex and you dont want your roommate to come in by accedent then just put a rubberband on the doorknob.
by voodoom August 20, 2006

Words related to rubberband on the doorknob

doorknob heads up rubberband rubber band sex warning