is the korean bero name for Gongchan from the korean group B1A4. They like to make fun of his constipatedness by calling him a poopsu, and chan was added to it. It can also be used for anyone ending or starting with Chan, like Chansung from 2PM. Used mostly for males.
You're such a chanpoopsu!
He's been in the washroom for so long, he's such a chanpoopsu!