To wear a belt with a loop buckle and stick the penis through for support, thus a holster. Preferably wear the belts that girls wear over their pants and not through the loops, they tend to have large rings for the buckle. Quite ideal for blowjobs or holding up large penis'.
"Dude that guy in the locker room was huge, he must wear a holster to keep his schlong from hanging out the bottom of his shorts."
"My girlfriend was having trouble keeping my dick in place when she was on top of me last night so she got her belt and put me in the holster."