This is the term used whenever something is done to someone in a harsh manner, whether it is playful or serious. A combination of the words, "got yo ass".
When you dunk on someone... "gotchoass"; When you prove somebody wrong..."gotchoass"; when you trick someone, hit someone, or anything else that excites you and upsets them..."GOTCHOASS"
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.