Someone who takes a long time to get something...i.e. he/she is slow. The word's short form is tubie...it literally means a florescent bulb... because when you switch it on the light doesnt actually flash until a few seconds later. Similarly, the person's brain whom you called a tubelight, doesn't light up and understand something until later
Rose is such a tubelight, she didn't get the joke until after everyone stopped laughing
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?"