Source : Webster's Revised Unabridged Dictionary (1913)

Mayfish \May"fish`\, n. (Zo["o]l.)
   A common American minnow ({Fundulus majalis}). See {Minnow}.

Source : WordNet®

mayfish
     n : black-barred fish of bays and coastal marshes of the
         Atlantic and Gulf coasts of the United States [syn: {striped
         killifish}, {may fish}, {Fundulus majalis}]