Related to methamphetamine, crank, gilly. If you've ever met a hardcoretweaker they'll report seeing monsters and other hallucinations after being up on the drug for days. Some call them the gillymonster.
fuck dude this one time i was up for 5 days straight and the fucking gillymonster was going to fucking get me!
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?"