Someone that stands behind another person and tries to give them instructions about how to squeeze a whitehead on their face.
I became very irritated when my mother stood behind me in front of the bathroom mirror as I was squeezing a pimple on my face. She kept telling me how to do it even after I told her to stop. I said, “Stop being a back seat pimple popper!”.
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?"