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.
by AZZA25 June 28, 2010