When having sexual intercourse with a woman and you come upon climax; instead of pulling out and shooting your wad in her face or guts, you shoot your load on the inside ie. "dumping in the box."
Cody: dude, i boned that blonde chick that works at the customer service desk.
Geoff: Did you pull out?
Cody: hell no bro i dumped in her box!!
Geoff: Fuckin sweet dude i'll take your sloppy seconds.