A chumsquare is a friend of yours who's pretty average. You know what kind of a day you're likely to spend with a chumsquare - nothing out of the blue or entertaining is likely to happen.
Families of chumsquares are common (we're pretty sure the chumsquare gene is passed on) and they generally watch alot of television in silence.
RB: So is Finn going clubbing with us tonight or not?
TM: Nah, he's set on staying home and being a chumsquare.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"