a person that has grown to adult size but whose mental capacity and attitude has not matured and developed appropriately. They often get on your nerves strutting around sharing their annoying irrational muttering and negative attitudes. Their character is tainted by the clash between their "know it all" stance and the fact that they really can not rationalize ideas & decisions on their own.
My supervisor is a real chidult, I wish he would grow up and get a life.
Please don't be a chidult, we really need mature adults on this team.