when a dude wears short underwear, and gets a boner. the head of his dick extends beyond the reach of the underwear and hardens. it has now become the only visible part of his dick and resembles when a cotton-mouth peeks above the water
guy 1: dude did you see jonathon give his speech?
guy 2: yea
guy 1: i didn't get to make it. how'd it go?
guy 2: i don't remember alot of it; i was distracted. his dick was just barely showing, like it was peeking out of his underwear
guy 1: oh no! he cotton-mouthed?!
guy 2: totally, we needa buy him some underwear that fit
guy 1: yea, cotton-mouthing sucks