What is Back It Up?
1.
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!"
See
