You've had anal sex and end up with a lot of shit on your penis, you have had a greasy night.
Brain: I heard you had a one night stand last night.
Chad: Yeah, but she was a proper minger, ended up having a greasy night!
Brain: That's fucking gross!
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?"