A selfie that has been taken by a person who is not part of the picture. A regular photo, basically.
Person #1: Hey, can you make a selfie of me and my gang?
Person #2: A someoneelsie.
Person #1: What?
Person 2: You are not taking the picture yourself, so it's not a "selfie". It's a someoneelsie.
Person #1: Ugh, fine. Just take the picture, okay?
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.