The Collaborative International Dictionary of English v.0.48:

Plug \Plug\, v. t. [imp. & p. p. Plugged; p. pr. & vb. n.
   Plugging.]
   To stop with a plug; to make tight by stopping a hole.
   [1913 Webster]

   2. To briefly publicize or advertise, especially during a
      public event not specifically intended for advertising
      purposes; as, during the interview he plugged his new
      book.

   Syn: put in a plug for.
        [PJC]

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.]
      [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]

WordNet (r) 3.0 (2006):

plug
    n 1: blockage consisting of an object designed to fill a hole
         tightly [syn: plug, stopper, stopple]
    2: a wad of something chewable as tobacco [syn: chew, chaw,
       cud, quid, plug, wad]
    3: blatant or sensational promotion [syn: ballyhoo, hoopla,
       hype, plug]
    4: electrical device that fits into the cylinder head of an
       internal-combustion engine and ignites the gas by means of an
       electric spark [syn: spark plug, sparking plug, plug]
    5: an electrical device with two or three pins that is inserted
       in a socket to make an electrical connection [syn: plug,
       male plug]
    6: an upright hydrant for drawing water to use in fighting a
       fire [syn: fireplug, fire hydrant, plug]
    7: an old or over-worked horse [syn: hack, jade, nag,
       plug]
    v 1: fill or close tightly with or as if with a plug; "plug the
         hole"; "stop up the leak" [syn: plug, stop up,
         secure]
    2: persist in working hard; "Students must plug away at this
       problem" [syn: plug, plug away]
    3: deliver a quick blow to; "he punched me in the stomach" [syn:
       punch, plug]
    4: make a plug for; praise the qualities or in order to sell or
       promote
    5: insert a plug into; "plug the wall"
    6: insert as a plug; "She plugged a cork in the wine bottle"