The Collaborative International Dictionary of English v.0.48:

Merman \Mer"man\, n.; pl. Mermen.
   The male corresponding to mermaid; a sea man, or man fish.
   [1913 Webster] Mero de lo alto
   Mero

WordNet (r) 3.0 (2006):

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