Fist weapons are those which are gripped in a fist, such as brass knuckles. Fist weapons use the melee "unarmed" weapon skill, so using a fist weapon will train up that skill. Therefore, fist weapons will not appear in your list of skills even when you can use them.
Though fist weapons use the "unarmed" weapon skill, this is different from fighting unarmed. Because they use the unarmed skill, a fist weapon wielder will still be able to do some damage when disarmed, though they still won't be able to use abilities that require a weapon.
Due to the small number of fist weapons in the game (all pre-wrath weapons are listed here) they can make a poor choice of weaponry before level 60. However, if you know where to look and are willing to put some extra time into running dungeons you should be able to find enough upgrades that this won't be a problem.
See the list of trainers for places to learn this weapon skill.
The Class proficiencies page lists which classes can use this type of weapon.
The Burning Crusade Fists Edit
Wrath of the Lich King Fists Edit