A gronkadonk is usually a crabby old man/woman that barks at children the way a dog barks at a mailman. Most often, it is directed at teachers that have been around since the late cretaceous period, and act as such.
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.