A COG terror is somebody who has bad centre of gravity (COG) from perhaps drunkenness or one leg being shorter than another. They pose a problem to other folk due to bad balance and to the fact that they may keep walloping into their acquaintance/friend.
John: God, Michael,do you remember last night at all?
Michael: Partly. Why?
John: Well when I walked you home in your drunk state, not only were you trying to make out with me but you were a total COG terror!