The line comes from Dragon Ball Z, where Vegeta reacts in disbelief to Goku's power level during their early rivalry. In the original Japanese version the reading was over 8000, but the English dub changed it to over 9000, and that wording stuck. A short clip spread widely on YouTube and forums in the mid 2000s, turning the shout into a universal shorthand for anything absurdly large or powerful. It remains a staple anime meme because the number is specific, quotable and easy to apply elsewhere.