The Collaborative International Dictionary of English v.0.48:

Halfbeak \Half"beak`\ (h[aum]f"b[=e]k), n. (Zool.)
   Any slender, marine fish of the genus Hemirhamphus, or of
   the family Hemiramphidae, having an elongated protruding
   lower jaw; -- called also balahoo.
   [1913 Webster]

WordNet (r) 3.0 (2006):

halfbeak
    n 1: tropical and subtropical marine and freshwater fishes
         having an elongated body and long protruding lower jaw