When an large scale attack occurs within the first 10 min or so of a RTS game while the opposing player is focusing on gathering/producing resources.
Originally from the RTS game Age of Empires, a tool rush is when a player attacks while still in the tool age. As the goal of any RTS game is to first increase production of resources to a high level and then increase production of battle units, one can attack an enemy within the first 10 min. of a game while the enemy has no or few units of attack and little defenses and is focusing all of their attention on gathering resources for which to build battle units and defenses.
Tool rushs must be carefully orchestrated as the rushing player will usually have few resources as well, and few, low-tech units for the attack.
Ok, 4 min into the game, I need some more wood. I'll send villagers over to this forest... <sounds of dying villagers> what the hell? AWW CRAP! ... tool rush?! CMON MAN!