Sans is a major character in Undertale, the 2015 RPG by Toby Fox, and appears again in Deltarune. In the game he hints that he remembers the player's actions across resets, though the game itself has no voice acting, so this line is a fan-made voice dub that caught on through soundboards and short-form edits. It spread alongside the Undertale AU and meme community, where creators needed a quick way to signal that a character had seen everything before. Its staying power comes from that familiar mix of casualness and menace.