Why is the ninth month called September?
late 14c., apparently from Old French dague "dagger," from Old Provençal dague or Italian daga, of uncertain origin; perhaps Celtic, perhaps from Vulgar Latin *daca "Dacian knife," from the Roman province in modern Romania. The ending is possibly the faintly pejorative -ard suffix. Attested earlier (1279) as a surname (Dagard, presumably "one who carried a dagger"). Middle Dutch dagge, Danish daggert, German Degen also are from French.
short stabbing knife, ostensibly the diminutive of the sword, though in ancient and medieval times the distinction between a long dagger and a short sword was often obscure. From approximately 1300 the European dagger was consistently differentiated from the sword; in the 16th century a school of fencing developed in which a specially designed dagger with a large guard was held in the left hand and used for parrying.