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

   Slug caterpillar. Same as Slugworm.
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