merman

Etymology

From Middle English mereman, from Old English *meremann (“merman”) (suggested by Old English meremenn, meremennen, meremenin (“mermaid, siren”)), equivalent to mer- + man. Cognate with Dutch meerman (“merman”), German Meermann (“merman”).

noun

1. A legendary creature, human male from the waist up, fishlike from the waist down.
   The meer man hath no feet but fins / And this hath legs you see and shins. 1653, The Loves of Hero and Leander: A Mock Poem, London, page 19