A dagger is a weapon, a sharp knife that's used for self-defense or fighting. During the Late Middle Ages, daggers were a sort of back-up to the longer sword, both carried during battles.
Most daggers have a sharp, pointed blade and a handle with a "crossguard" that protects a fighter's hand from sliding forward and getting cut. Throughout history, various kinds of daggers have been used in close combat. The word comes from the Old French dague, though its origin beyond that isn't known. The ancient last name Dagard, dating from the 13th century or earlier, meant "one who carries a dagger."