Mettaton EX is a character from Undertale, the 2015 indie role-playing game by Toby Fox, where he appears as a flamboyant robot entertainer and boss opponent. The Yeah is one of the vocal reactions heard when he is struck during that boss fight, and fans later clipped it for soundboards and video edits. It spread because it is instantly recognisable to Undertale players, short enough to drop into almost any clip, and funny precisely because Mettaton treats damage like part of a show.