Talking in one's sleep, usually in a nonsensical manner. It's possible to have a multiple-sentence "conversation" with someone who is "ferdering". The ferderer will not remember the conversation.
Etymology: Some English-speaking people who talk in their sleep say words that sound like "ferder ferder ferder".
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?"