Webster's Revised Unabridged Dictionary · 1913

A fully automatic rapid-firing rifle, which continues to fire bullets repeatedly as long as the trigger is depressed; lighter versions may be carried in the hands, and heavier versions may be mounted on a tripod, vehicle, or other mount. The lighweight versions are sometimes called a submachine gun.

Occurring in rapid succession, like the firing of a machine gun; as, Tom was a persuasive speaker, with a smooth deep voice, polysyllabic vocabulary, and a machine-gun articulation that overwhelmed listeners.