The Collaborative International Dictionary of English v.0.48:

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.]
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   3. A high, tapering silk hat. [Slang, U.S.]
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   4. A worthless horse. [Slang, U.S.]
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   5. (Building) A block of wood let into a wall, to afford a
      hold for nails.
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   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]