The Collaborative International Dictionary of English v.0.48:

Hairtail \Hair"tail`\ (-t[=a]l`), n. (Zool.)
   Any species of marine fishes of the genus Trichiurus; esp.,
   Trichiurus lepturus of Europe and America. They are long
   and like a band, with a slender, pointed tail. Called also
   bladefish.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hairtail
    n 1: long-bodied marine fishes having a long whiplike scaleless
         body and sharp teeth; closely related to snake mackerel
         [syn: cutlassfish, frost fish, hairtail]