To masturbate prior to making a decision involving women. "Fap" comes from the onomatopoeic representation of masturbation popularized on the internet via web forums.
When one faps on it, sexual tension is released through masturbation, resulting in a mind that is clear in thought and a rational decision can be made involving women, usually sexual decisions.
Bro 1: I'm not sure I want to drop $40 tonight on drinks with Jen at the bar. I wanna fuck her, but I don't know if it's worth spending dough on her.
Bro 2: Fap on it. Then you'll know what to do.