Source : Webster's Revised Unabridged Dictionary (1913)

Swimming \Swim"ming\, a.
   1. That swims; capable of swimming; adapted to, or used in,
      swimming; as, a swimming bird; a swimming motion.

   2. Suffused with moisture; as, swimming eyes.

   {Swimming bell} (Zo["o]l.), a nectocalyx. See Illust. under
      {Siphonophora}.

   {Swimming crab} (Zo["o]l.), any one of numerous species of
      marine crabs, as those of the family {Protunid[ae]}, which
      have some of the joints of one or more pairs of legs
      flattened so as to serve as fins.

Source : WordNet®

swimming crab
     n : marine crab with some legs flattened and fringed for
         swimming