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The rapping of one's knuckles across perky breasts-a way to please yourself by drumming your hands on a girl's front. Arousal is usually expected.
Matt: "So how'd it go with Jack last night?"

Danielle: "Well, we had a bit of a knock-knock fest, and now I'm all sore. But he's in a better mood than ever!"

Matt: "Okay, then."
by Lord Blackness March 29, 2014
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