a swellcum sight

Refers to any delightful and sexual-desire-arousing visual experience --- like a group of hot chicks sunning themselves topless --- that causes a guy to get a woody and jizz his underwear.

"Relieving the pressure" before heading out the door can help you avoid any embarrassing "oopsies" if you unexpectedly happen upon a swellcum sight while you're "out and about".