The line comes from James Cameron's Titanic from 1997, where the ship's lookout spots the iceberg and shouts the warning to the bridge moments before the collision. Because the film is so widely known, the single shouted phrase is instantly recognisable even out of context, and it has become shorthand online for a disaster that everyone can see coming except the person heading towards it. Editors reuse it in short videos, reaction clips and gaming montages because it delivers the whole joke of impending doom in under two seconds.