A tardnuckle is a hybrid of someone who acts retarded, and a knucklehead. Usually someone who enjoys playing retarded for fun and looks like a vacuous goon doing so.
Mike "I'm totally going to act full retard at the party. "
John "You would because you're such a tardnuckle."
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?"