Top Definition
What a guy says to a girl he's fucking from behind - asking her to push her ass back so she grinds onto him.

Also, in lesser used context, it means a girl has a nice ass.
*A couple doing it doggy style*

Chrissy: "Oh that feels so good!"
Tom: "Oh yeah, you want it harder?"
Chrissy: "Yes!"
Tom: "Back it up, back it up!"
by TJackson July 12, 2008
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