A bayonet is a knife attached to the end of a rifle. This handy extension to the business end of a rifle gets its name from the French city of Bayonne. It's thought to have developed when French peasantry ran out of ammo and stuffed their knives into the muzzles of their muskets. A bayonet makes the rifle more versatile: besides shooting people at a distance, it's now more deadly in hand-to-hand combat. With a bayonet, you've got a gun and a knife in one package.

Source: Vocabulary.com