to masturbate. Basically to "prepare to shower". It is a lot more convenient to masturbate and jizz all over yourself before you shower so you can wash it off. Or in a girls case... I don't know maybe they just like to do it before they get in the shower, which is extremely sexy for some reason.
I'm not allowed to shower prep for two weeks because my girlfriend wants me to save it for her. Wow she is horny.