The horn comes from Mann vs Machine, the cooperative mode in Valve's Team Fortress 2 in which players defend a base against waves of robots, including a slow, bomb-carrying tank whose arrival the horn announces. As TF2 clips and memes spread on video platforms and soundboards, the short blast became shorthand for an incoming threat, even outside TF2 videos. It remains popular because TF2 players recognise it instantly, and because, like a danger alarm, it tells viewers something large and hard to stop is on its way.