The Collaborative International Dictionary of English v.0.48:

Limpet \Lim"pet\ (l[i^]m"p[e^]t), n. [Prob. through French fr.
   L. lepas, -adis, Gr. lepa`s, -a`dos.] (Zool.)
   1. In a general sense, any hatshaped, or conical, gastropod
      shell.
      [1913 Webster]

   2. Any one of many species of marine shellfish of the order
      Docoglossa, mostly found adhering to rocks, between
      tides.
      [1913 Webster]

   Note: The common European limpets of the genus Patella
         (esp. Patella vulgata) are extensively used as food.
         The common New England species is Acm[ae]a
         testudinalis. Numerous species of limpets occur on the
         Pacific coast of America, some of them of large size.
         [1913 Webster]

   3. Hence: Somthing or someone that clings tenaciously to
      another object or person; specifically a military
      explosive device having magnets allowing it to cling to a
      metallic target object, such as the hull of a ship.
      [PJC]

   3. Any species of Siphonaria, a genus of limpet-shaped
      Pulmonifera, living between tides, on rocks.
      [1913 Webster]

   4. A keyhole limpet. See Fissurella.
      [1913 Webster]