The act of procrastinating, even if it means doing something you usually would not want to do ie. doing housework, feeding pets etc. Named after a popular Hagaren character, Roy mustang for obvious reasons.
Hawkeye: Sir, please stop cleaning the windows and get back to work.
Mustang: I am working! I'm.. MUSTANGING!
Hawkeye: ... You. Work. Now.
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?"