The Collaborative International Dictionary of English v.0.48:

Dab \Dab\, n. [Perh. so named from its quickness in diving
   beneath the sand. Cf. Dabchick.] (Zool.)
   A name given to several species of flounders, esp. to the
   European species, Pleuronectes limanda. The American rough
   dab is Hippoglossoides platessoides.
   [1913 Webster]