The Collaborative International Dictionary of English v.0.48:

Lobster \Lob"ster\, n. [AS. loppestre, lopystre prob., corrupted
   fr. L. locusta a marine shellfish, a kind of lobster, a
   locust. Cf. Locust.] (Zool.)
   1. Any large macrurous crustacean used as food, esp. those of
      the genus Homarus; as the American lobster (Homarus
      Americanus), and the European lobster (Homarus
      vulgaris). The Norwegian lobster (Nephrops Norvegicus)
      is similar in form. All these have a pair of large unequal
      claws. The spiny lobsters of more southern waters,
      belonging to Palinurus, Panulirus, and allied genera,
      have no large claws. The fresh-water crayfishes are
      sometimes called lobsters.
      [1913 Webster]

   2. As a term of opprobrium or contempt: A gullible, awkward,
      bungling, or undesirable person. [Slang]
      [Webster 1913 Suppl.]

   Lobster caterpillar (Zool.), the caterpillar of a European
      bombycid moth (Stauropus fagi); -- so called from its
      form.

   Lobster louse (Zool.), a copepod crustacean (Nicotho["e]
      astaci) parasitic on the gills of the European lobster.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lobster
    n 1: flesh of a lobster
    2: any of several edible marine crustaceans of the families
       Homaridae and Nephropsidae and Palinuridae

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "lobster":
   Chilopoda, Chordata, Dungeness crab, Echiuroidea, Ectoprocta,
   Entoprocta, Japanese crab, Monoplacophora, Nemertinea, Phoronidea,
   blue point, clam, coquillage, crab, crawdad, crawfish, crayfish,
   gawk, klutz, langouste, limpet, littleneck clam, looby, lout,
   lubber, lug, lummox, lump, mussel, oaf, oyster, palooka,
   periwinkle, prawn, quahog, scallop, shellfish, shrimp, snail,
   soft-shell crab, steamer, whelk