This commonly occurs when you lean back and put your legs up, while in a seated position and you're rockin short, loose-fitting bottoms, i.e., short shorts or short swim trunks.
Being stoned helps, too.
This occurs with a much higher degree of frequency in wearers of boxers than in wearers of more supportive undergarments.
the means in wanting to "get some" or have some sort of sexual activity with the opposite sex
guy2: aw jus hanging brain man.
guy1: what do you wanna do mayne?
guy2: all i know, is i wanna hang brain!
guy1: k lets call some girls!