It's a insult used on a person who is a pain or useless. The insult reffers to when a male ejaculates on his pants and a stain is left where the semen (or in slang cum fell. Hence the use of "cum") . It's acronym form is CSOTPOS. Used by many teenagers and is not understood by most teaches so it is higly useful.
Jim: DAVE DAVE DAVE!
Dave: Shut up you cum stain on the pants of society
Someone who is so physically attractive that you would literally have sex with them on the floor....or in a closet....or basically anywhere else.
A:"OMG, have you seen jack's brother? He's so cute!!"
B:"Cute like little kid cute or cute like hot..."
A:"More like get-on-the-floor-take-off-your-pants hot"
B:"Damn."