Warcraft 3, Blizzard's real-time strategy game released in 2002, gave its human peasant workers a set of short spoken responses for moving, working, and being selected. 'A trabajar' comes from the Spanish-language version, where the peasant acknowledges a work order in his own language. As gameplay clips, streams, and Spanish-speaking gaming communities shared those unit quotes, the line became a handy shorthand for starting a task. It endures because it is instantly understood, even by players who do not speak Spanish, as the sound of getting down to work.