Top Definition
Said about a sexually appealing woman, with whom you would be very eager to have sex.

To explain the metaphor: you may be a little too vigorous, and push your penis all the way through her, hence, knocking the back out of her... Obviously not physically possible, but a good, although rather crude, metaphor
Guy A: Phwoar, Check her out, what a stunner!
Guy B: God yeah, I'd knock the back out of that!
by Dr Mox April 07, 2007

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