"edged or pointed weapon for thrusting, shorter than a sword," late 14c. (mid-14c. in Anglo-Latin), apparently related to Old French dague "dagger," from Old Provençal or Italian daga, which are of uncertain origin; perhaps from Celtic, perhaps from Vulgar Latin *daca "Dacian knife," from the Roman province (see Dacian). The ending is possibly the faintly pejorative -ard suffix.
Attested earlier (1279) as a surname (Dagard, presumably "one who carried a dagger"). Also compare dogwood. Middle Dutch dagge, Danish daggert, German Degen also are from French. By 16c.-17c. swordsmen held it in the left hand to parry thrusts from the opponent's rapier. As "a reference mark in the form of a dagger," by 1706.