A squirtbox is a cardboard box that is taped shut with a hole cut into it that one would use to masturbate into. This could have a picture of someone's face/genitals taped to it to add to the affect.
"HeyJim, where's your squirtbox? I need to crank one out!"
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?"