Weaponsmith is the skill associated with the weaponsmithing labor. Weaponsmiths make metal weapons (including crossbows), trap components, picks and bolts at a metalsmith's forge. The skill level of the weaponsmith determines the quality of the produced weapons, as well as the speed of their production.
Due to the necessity of weaponry in most forts and the high added value of weapons (especially quality ones) compared to their base material, a high-skill weaponsmith is a precious dwarf indeed. Do not let him go in the caverns and be eaten by some random giant cave spider.