The line comes from the 1985 film Back to the Future, where Doc Brown, played by Christopher Lloyd, explains that his DeLorean time machine needs 1.21 gigawatts of electricity to work, power he hopes to capture from a lightning strike. The exact figure and his urgent delivery made it one of the film's most quoted moments, and it later spread through internet memes, reaction GIFs and short video edits. It endures because it turns any large quantity into an instantly understood joke about impossible power.