The Collaborative International Dictionary of English v.0.48:

Prawn \Prawn\, n. [OE. prane, of unknown origin; cf. L. perna a
   sea mussel.] (Zool.)
   Any one of numerous species of large shrimplike Crustacea
   having slender legs and long antenn[ae]. They mostly belong
   to the genera Pandalus, Pal[ae]mon, Pal[ae]monetes, and
   Peneus, and are much used as food. The common English prawn
   is Pal[ae]mon serratus.
   [1913 Webster]

   Note: The name is often applied to any large shrimp.
         [1913 Webster]