Source : Webster's Revised Unabridged Dictionary (1913)

Merman \Mer"man\, n.; pl. {Mermen}.
   The male corresponding to mermaid; a sea man, or man fish.

Source : WordNet®

Merman
     n 1: United States singer who appeared in several musical
          comedies (1909-1984) [syn: {Ethel Merman}]
     2: half man and half fish; lives in the sea