Brass knuckles, an item that increases damage done with bare hands and martial arts if you are unarmed. There are multiple takes on it:

1. They are a ring-slot item, possibly occupying both ring slots at once, or just one. In this case, you must also not be wearing gloves in order to get the bonus damage, or else you can’t even put on gloves while wearing the knuckles. Can be charged like other rings to increase the damage dealt by them; however, the damage boost even when they are +0 is significant.
2. They are glove-slot armor. This is nice because it means they can be enchanted with enchant armor rather than charging. While the enchantment increases damage, it does NOT provide any AC bonus.