The Rice University Neologisms Database

Hyde means someone’s, usually unwanted, alter ego. Hyde is used mostly as a pro-noun when referring to someone who is acting out or acting in a completely different manner as they usually do. Hyde can also be used to refer to the alter ego itself as a separate entity to explain that someone’s alter ego is coming out or is being released. Hyde is usually expressed in a negative way because that alter ego is unattractive to other around that person.