The Collaborative International Dictionary of English v.0.48:

Sea slug \Sea" slug`\ (Zool.)
   (a) A holothurian.
   (b) A nudibranch mollusk.
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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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The Collaborative International Dictionary of English v.0.48:

Trepang \Tre*pang"\, n. [Malay tr[imac]pang.] (Zool.)
   Any one of several species of large holothurians, some of
   which are dried and extensively used as food in China; --
   called also b[^e]che de mer, sea cucumber, and sea
   slug. [Written also tripang.]
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   Note: The edible trepangs are mostly large species of
         Holothuria, especially Holothuria edulis. They are
         taken in vast quantities in the East Indies, where they
         are dried and smoked, and then shipped to China. They
         are used as an ingredient in certain kinds of soup.
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WordNet (r) 3.0 (2006):

sea slug
    n 1: any of various marine gastropods of the suborder
         Nudibranchia having a shell-less and often beautifully
         colored body [syn: sea slug, nudibranch]