A room in the after-part of a lower gun-deck for the
accommodation of junior officers. GUN PHRASES.
He's a great gun.
A man of note. Son of a gun.
A jovial fellow. Sure as a
Quite certain. It is as certain to happen as a gun to go off
if the trigger is pulled.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894