The term "Zerg Rush" is based on the term "Grunt Rush" from Warcraft II. A Grunt Rush is when you build your Barracks first before your Town Hall to start churning out Grunts as fast as possible. It's a degenerate strategy that's pretty much indefensible. The term was coined by a player named Tou in 1996. The Zerg race in StarCraft were particularly good at attacking with many units early in the game, so this term was coopted by StarCraft players and it became "Zerg Rush."