The Collaborative International Dictionary of English v.0.48:

Slug \Slug\, v. i.
   To move slowly; to lie idle. [Obs.]
   [1913 Webster]

         To slug in sloth and sensual delight.    --Spenser.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slug \Slug\, v. t.
   To make sluggish. [Obs.] --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slug \Slug\, v. t. [imp. & p. p. Slugged; p. pr. & vb. n.
   Slugging.]
   1. To load with a slug or slugs; as, to slug a gun.
      [1913 Webster]

   2. To strike heavily. [Cant or Slang]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slug \Slug\, v. i.
   To become reduced in diameter, or changed in shape, by
   passing from a larger to a smaller part of the bore of the
   barrel; -- said of a bullet when fired from a gun, pistol, or
   other firearm.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slug \Slug\, n. [OE. slugge slothful, sluggen to be slothful;
   cf. LG. slukk low-spirited, sad, E. slack, slouch, D. slak,
   slek, a snail.]
   1. A drone; a slow, lazy fellow; a sluggard. --Shak.
      [1913 Webster]

   2. A hindrance; an obstruction. [Obs.] --Bacon.
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   3. (Zool.) Any one of numerous species of terrestrial
      pulmonate mollusks belonging to Limax and several related
      genera, in which the shell is either small and concealed
      in the mantle, or altogether wanting. They are closely
      allied to the land snails.
      [1913 Webster]

   4. (Zool.) Any smooth, soft larva of a sawfly or moth which
      creeps like a mollusk; as, the pear slug; rose slug.
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   5. A ship that sails slowly. [Obs.] --Halliwell.
      [1913 Webster]

            His rendezvous for his fleet, and for all slugs to
            come to, should be between Calais and Dover.
                                                  --Pepys.
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   6. [Perhaps a different word.] An irregularly shaped piece of
      metal, used as a missile for a gun.
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   7. (Print.) A thick strip of metal less than type high, and
      as long as the width of a column or a page, -- used in
      spacing out pages and to separate display lines, etc.
      [1913 Webster]

   Sea slug. (Zool.)
      (a) Any nudibranch mollusk.
      (b) A holothurian.

   Slug caterpillar. Same as Slugworm.
      [1913 Webster]

WordNet (r) 3.0 (2006):

slug
    n 1: a projectile that is fired from a gun [syn: bullet,
         slug]
    2: a unit of mass equal to the mass that accelerates at 1
       foot/sec/sec when acted upon by a force of 1 pound;
       approximately 14.5939 kilograms
    3: a counterfeit coin
    4: an idle slothful person [syn: sluggard, slug]
    5: an amount of an alcoholic drink (usually liquor) that is
       poured or gulped; "he took a slug of hard liquor"
    6: a strip of type metal used for spacing [syn: type slug,
       slug]
    7: any of various terrestrial gastropods having an elongated
       slimy body and no external shell
    8: (boxing) a blow with the fist; "I gave him a clout on his
       nose" [syn: punch, clout, poke, lick, biff, slug]
    v 1: strike heavily, especially with the fist or a bat; "He
         slugged me so hard that I passed out" [syn: slug, slog,
         swig]
    2: be idle; exist in a changeless situation; "The old man sat
       and stagnated on his porch"; "He slugged in bed all morning"
       [syn: idle, laze, slug, stagnate] [ant: work]

Moby Thesaurus II by Grady Ward, 1.0:

209 Moby Thesaurus words for "slug":
   Chilopoda, Chordata, Echiuroidea, Ectoprocta, Entoprocta,
   Intertype, Linotype, Monoplacophora, Monotype, Nemertinea,
   Phoronidea, baggage check, ball, bang, bar shot, bash, bat,
   beating, belt, biff, bird shot, blast, blow, bonk, buckshot,
   bullet, bum, cannon shot, cannonball, case shot, check, chop, clap,
   clip, clobber, clock watcher, clout, clump, cold-type typesetting,
   coldcock, composing, composing stick, composition,
   computerized typesetting, counter, coupon, crack, crossbar shot,
   cut, dallier, dash, dawdle, dawdler, deal, deal a blow, deck,
   diddler, dig, dillydallier, dint, do-nothing, dolittle, doodler,
   dose, dram, drone, drop, drub, drubbing, drumming, duck shot,
   dumdum bullet, dummy, em, em quad, en, en quad, expanding bullet,
   faineant, fetch, fetch a blow, five-em space, foot-dragger,
   four-em space, furniture, fusillade, galley chase,
   gentleman of leisure, goldbrick, goldbricker, goof-off, grape,
   grapeshot, hair space, hat check, hit, hit a clip,
   hot-metal typesetting, idler, imposition, jab, jolt, justification,
   justification space, justifying space, knock, knock cold,
   knock down, knock out, laggard, langrel shot, layout, lazybones,
   let have it, lick, lie-abed, line of type, lingerer, loafer,
   loiterer, loller, lotus-eater, lounger, lubber, manstopping bullet,
   mope, moper, nip, paste, patent space, pellet, pelt,
   photocomposition, photosetting, phototypesetter,
   phototypesetting machine, plodder, plunk, poke, portion, potterer,
   pound, procrastinator, punch, putterer, quad, quadrat, quoin, rap,
   rifle ball, round shot, scrip, setting, shell, shot, shrapnel,
   slacker, slam, sleepyhead, slog, slouch, sloucher, sloven,
   slow goer, slow-foot, slowbelly, slowpoke, slugabed, sluggard,
   smack, smash, smite, snail, snap, snifter, snort, soak, sock,
   space, spaceband, split shot, stick-in-the-mud, strike, strike at,
   stroke, swat, swing, swipe, tag, tattoo, thick space, thin space,
   thump, thwack, ticket, time killer, time waster, token, tortoise,
   tot, trifler, typesetting, typesetting machine,
   waiter on Providence, wallop, whack, wham, whop, yerk