Slang terminology for a gun. Many people thing the term is short for the
gatling gun which was an 1860s multi fire rapid
machine gun. Around the late 1800s to turn of the century, the term gatlin became term for a revolver.
It was not until the 1939 when the crime/mystery author Raymond Chandler detective character Philip Marlowe used the term gat to refer to any kind of pistol or small hand held gun, in the novel
The Big Sleep.