A brainworm is an idea, song, or conspiracy theory that gets stuck in your head and changes how you see the world. If your teen says someone has "YouTube brainworms" they mean that person has been radicalised by what they watch online. Sometimes mildly concerning: it implies someone's media diet is distorting their thinking. Worth a conversation about media literacy. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.