Originally: a machine-gun with a number of barrels, able to discharge projectiles rapidly and simultaneously in large quantities, or singly in quick succession. Later generally: any machine-gun.
The original mitrailleuse was first brought into service in the Franco-Prussian War of 1870–1.
Mid 19th century; earliest use found in Scientific American. From French mitrailleuse rapid-fire automatic weapon from mitrailler + -euse.