When someone does something usually really stupid, or says something stupid, or makes no sense at all, then you give them a neck which is slapping the back of their neck hard (depending on what they did/say... if it's real stupid then you give it to them harder)
Garrick: Jennifer Alba looks hella good
Conan: you mean Jessica Alba?
Garrick : is that her name?
Conan : GIVE ME THAT NECK !
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.