Casing
Or casing sausage, is when you get out of the pool and your shorts are sticking to your legs so tight that it looks like they vacuumed sealed your penis, and it becomes a very visible outline.
Girl at the pool: Hey, do you see that guy over there?
Friend: Yeah, why?
Girl at the pool: Look at his shorts, he is totally casing.
Friend: Yeah, look at it, its pretty big.