Idiot mittens - a term, as far as I know, coined by the excellent Richard Digance. Idiot mittens have string connecting them: Siamese mittens, I guess. You pass one mitten up the sleeve of your coat (and, if you're wearing idiot mittens, then it's more than likely going to be a snorkel jacket) and then down the other sleeve. When you put the coat on, the mittens dangle appealingly (or appallingly, I suppose, depending on your perspective and level of cynicism)just beneath your hands.
Why do it? Because, once they're connected and strung, they are practically impossible to lose, whether you've got your hands in them or not. What sort of person would lose mittens? (Or more to the point, what sort of person would wear them in the first place?) - Only an idiot, surely?
Hence the name. All very logical.
The best way to abuse someone wearing idiot mittens is to grab them by one hand and pull. They will undoubtedly whack themselves in the face with their other hand. Very stupid people who wear them, and who wears them if they're intelligent(?), can hit themselves in the face just by waving to their friends. If their friends are wearing them too, they'll hit themselves in the face by default. What fun!