The shout comes from DreamWorks' animated film The Croods, released in 2013, where the Crood family collide with Guy, a clever newcomer full of ideas. At one point Guy escapes from Grug, the family's overprotective father, and the cry of 'He's loose!' rings out in comic panic. The line lived on as a soundboard favourite, passed around on forums and meme pages as a reaction for escapes and near-misses. Its staying power is simple: two words that turn any disappearing act into a joke.