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"

Moby Thesaurus II by Grady Ward, 1.0:

253 Moby Thesaurus words for "plug":
   PR, acknowledgment, advert, advertise, advertisement, advocate,
   appreciation, back, bait, balker, balky horse, ballyhoo, bar, bark,
   bill, bind, birdlime, block, block up, blockade, blurb, bola,
   boost, bright light, build up, buildup, bulletin, bung, caulk,
   cavendish, celebrity, charge, check valve, chew, chink, choke,
   choke off, choke up, chug on, circularize, clog, clog up, close,
   close off, close up, cobweb, cock, commend, commendation,
   commercial, commercial announcement, common knowledge, congest,
   constipate, cork, cover, crock, crowbait, cry, cry up, currency,
   dam, dam up, daylight, detonate, dig, discharge, dog, dragnet,
   drop, drudge, eclat, eject, establish, exposure, fag, fame,
   famousness, faucet, fell, fill, fill up, fire, fire hydrant,
   fire off, fireplug, fishhook, fly, foul, garron, gill net,
   give a write-up, give publicity, glare, goat, good word, grind,
   ground bait, grub, gun, gun for, hack, hammer, hammer away, hit,
   honorable mention, hook, hoopla, hue and cry, hydrant, hype, jade,
   jam, jig, jog on, jughead, keep doggedly at, lariat, lasso,
   let fly, let off, lid, lime, limelight, load, lumber, lure,
   maximum dissemination, mention, meshes, moil, nag, net, noose,
   notoriety, obstipate, obstruct, occlude, pack, peg, peg away, pelt,
   pepper, pick off, pigtail, pin, pistol, placard, plod, plod along,
   plug along, plug at, plug away, plug up, post, post bills, post up,
   pot, potshoot, potshot, pound away, pound net, press notice,
   press-agent, prime, promote, promotion, public eye,
   public knowledge, public relations, public report, publicity,
   publicity story, publicize, publicness, puff, puffing, purse seine,
   push, quid, reclame, recognition, recommend, recommendation,
   report, riddle, roarer, rogue, rosinante, rub on, scalawag, schlep,
   sea cock, seal off, seal up, seine, sell, shoot, shoot at,
   shoot down, slog, slog away, snare, sniggle, snipe,
   speak highly of, speak warmly of, speak well of, spiel, spigot,
   spike, spile, spill, spinner, spot, spot announcement, spotlight,
   springe, squid, stanch, stay, stench, stiff, stop, stop up,
   stopgap, stopper, stopple, strike, stuff, stuff up, stump, support,
   take a potshot, tap, toil, toils, torpedo, tout, tramp, travail,
   trawl, trudge, twist, valve, wad, wade through, water plug,
   whistler, wobbler, work away, write up, write-up