What does "plug" mean?

Additional senses

1. 2.A high, tapering silk hat. [Slang, U.S.]
2. 3.A worthless horse. [Slang, U.S.]
3. 4.(Building) A block of wood let into a wall, to afford a hold for nails. 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.
4. 5.To stop with a plug; to make tight by stopping a hole.