This is a mispronunciation of the word "funny." It is used mainly by trendy fucks that are trying way too hard to be cool. These are the same fools that use abbreviations like "lol" and "wtf" in everyday spoken conversation.
Guy: LOL! The Hangover was so funnay!
Other guy: God damn it! You fool!
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?"