Definition from Wiktionary, the free dictionary
The term originates in the Dungeons & Dragons (D&D) game, where it originally referred to a Githyanki fighter/wizard combination.
gish (plural gishes)
- (role-playing games) A magician, or character that is skilled in both physical combat and the use of magic. Most gish characters use their magical abilities to increase their own personal combat abilities (known as "buffing").
- (slang) An outsider.