What is 12-o-clock Tuck?
When you need to hide an erection and you tuck it straight up into your waistband/beltline. The cock is similar to the hand of a clock at the 12-o-clock position.
guy: Hey, can you tell I have a raging hard on right now?
girl: What the fuck?
guy: Yeah, just did a little 12-o-clock tuck.