masturbation done for the purpose of getting over
lust for
someone. The "it" in "rub it off" is the sexual attraction.
The term is similar to brush it off, but is explicitly sexual.
If a
hater talks shit to you, you can brush it off and get on with your life. If a bad bitch is stuck in your head, you can't brush it off: You gotta rub it off.
Holy shit! That
girl on the airplane was so
hot! I can't think knowing a chick that fine exists!
Dude, you're never going to see her again. Rub it off and get on with your life.