The phrase comes from George of the Jungle, the vine-swinging hero who regularly crashes into trees instead of clearing them, making watch out for that tree his running joke. The loud, isolated warning became popular online as a short meme sound, shared on soundboards and reused in fail compilations, gaming clips and reaction edits. It endures because the setup is instantly clear: a confident run, a shouted warning, and an accident the audience can already see coming.