The Collaborative International Dictionary of English v.0.48:

Snail \Snail\ (sn[=a]l), n. [OE. snaile, AS. sn[ae]gel, snegel,
   sn[ae]gl; akin to G. schnecke, OHG. snecko, Dan. snegl, Icel.
   snigill.]
   1. (Zool.)
      (a) Any one of numerous species of terrestrial
          air-breathing gastropods belonging to the genus Helix
          and many allied genera of the family Helicidae. They
          are abundant in nearly all parts of the world except
          the arctic regions, and feed almost entirely on
          vegetation; a land snail.
      (b) Any gastropod having a general resemblance to the true
          snails, including fresh-water and marine species. See
          Pond snail, under Pond, and Sea snail.
          [1913 Webster]

   2. Hence, a drone; a slow-moving person or thing.
      [1913 Webster]

   3. (Mech.) A spiral cam, or a flat piece of metal of spirally
      curved outline, used for giving motion to, or changing the
      position of, another part, as the hammer tail of a
      striking clock.
      [1913 Webster]

   4. A tortoise; in ancient warfare, a movable roof or shed to
      protect besiegers; a testudo. [Obs.]
      [1913 Webster]

            They had also all manner of gynes [engines] . . .
            that needful is [in] taking or sieging of castle or
            of city, as snails, that was naught else but hollow
            pavises and targets, under the which men, when they
            fought, were heled [protected], . . . as the snail
            is in his house; therefore they cleped them snails.
                                                  --Vegetius
                                                  (Trans.).
      [1913 Webster]

   5. (Bot.) The pod of the sanil clover.
      [1913 Webster]

   Ear snail, Edible snail, Pond snail, etc. See under
      Ear, Edible, etc.

   Snail borer (Zool.), a boring univalve mollusk; a drill.

   Snail clover (Bot.), a cloverlike plant (Medicago
      scuttellata, also, Medicago Helix); -- so named from
      its pods, which resemble the shells of snails; -- called
      also snail trefoil, snail medic, and beehive.

   Snail flower (Bot.), a leguminous plant (Phaseolus
      Caracalla) having the keel of the carolla spirally coiled
      like a snail shell.

   Snail shell (Zool.), the shell of snail.

   Snail trefoil. (Bot.) See Snail clover, above.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Phaseolus caracalla
    n 1: perennial tropical American vine cultivated for its racemes
         of showy yellow and purple flowers having the corolla keel
         coiled like a snail shell; sometimes placed in genus
         Phaseolus [syn: snailflower, snail-flower, snail
         flower, snail bean, corkscrew flower, Vigna
         caracalla, Phaseolus caracalla]