Shield user

From DwarfFortressWiki

Shield User is the skill associated with the use of shields and bucklers in combat. The skill is gained every time a block roll is attempted, and presumably increases the chance of higher block rolls. If the block roll succeeds the attack is blocked completely.

At legendary or near-legendary levels, this skill provides huge amounts of protection, allowing a dwarf with a decent shield to block many crossbow bolts, or even Dragonfire.