The Thompson is an American submachine gun, invented by John T. Thompson in 1919 that became infamous during the Prohibition era. It was a common sight of the time, being used by both law enforcement officers and criminals. The Thompson was also known informally as: the "Tommy Gun", the "Trench Broom", the "Trench Sweeper", the "Chicago Piano", the "Chicago Typewriter", and the "Chopper".
The M1921 was famous throughout its career with police and criminals and in motion pictures. This model gained fame from its' use by criminals during Prohibition, and was nicknamed 'tommy gun' by the media.
https://en.wikipedia.org/wiki/Tommy_gun