'Breech plug' definitions:

Definition of 'Breech plug'

From: GCIDE

Plug \Plug\, n. [Akin to D. plug, G. pflock, Dan. pl["o]k, plug, Sw. plugg; cf. W. ploc.]
1. Any piece of wood, metal, or other substance used to stop or fill a hole; a stopple. [1913 Webster]
2. A flat oblong cake of pressed tobacco. [U. S.] [1913 Webster]
3. A high, tapering silk hat. [Slang, U.S.] [1913 Webster]
4. A worthless horse. [Slang, U.S.] [1913 Webster]
5. (Building) A block of wood let into a wall, to afford a hold for nails. [1913 Webster]
6. An act of plugging[6]; a brief mention for the sake of publicity or advertisement, especially during a public event not specifically intended for advertising purposes; as, he put in a plug for his favorite charity. [PJC]
Breech plug (Gun.), in breech-loading guns, the metal plug or cylinder which closes the aperture in the breech, through which the gun is loaded.
Fire plug, a street hydrant to which hose may be attached. [U. S.]
Hawse plug (Naut.), a plug to stop a hawse hole.
Plug and feather. (Stone Working) See Feather, n., 7.
Plug centerbit, a centerbit ending in a small cylinder instead of a point, so as to follow and enlarge a hole previously made, or to form a counterbore around it.
Plug rod (Steam Eng.), a rod attached to the beam for working the valves, as in the Cornish engine.
Plug valve (Mech.), a tapering valve, which turns in a case like the plug of a faucet. [1913 Webster]