The sound comes from the opening riff of "Bad to the Bone" by George Thorogood and the Destroyers, released in 1982. Online, that riff became closely tied to skull reveal edits and parody badass moments, especially in short meme videos and reposted reaction clips. Its popularity lasted because the audio is immediately recognisable and the joke is easy to understand in a second: ordinary scene, sudden skull, instant attitude. It fills a similar reaction role to bruh, but with a musical punch instead of a spoken one.