Plug n.
 1. Any piece of wood, metal, or other substance used to stop or fill a hole; a stopple.
 2. A flat oblong cake of pressed tobacco. [U. S.]
 3. A high, tapering silk hat. [Slang, U.S.]
 4. A worthless horse. [Slang, U.S.]
 5. Building A block of wood let into a wall, to afford a hold for nails.
 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.