The Collaborative International Dictionary of English v.0.48:

Humpback \Hump"back`\, n. [Cf. Hunchback.]
   1. A crooked back; a humped back. --Tatler.
      [1913 Webster]

   2. A humpbacked person; a hunchback.
      [1913 Webster]

   3. (Zool.)
      (a) Any whale of the genus Megaptera, characterized by a
          hump or bunch on the back. Several species are known.
          The most common ones in the North Atlantic are
          Megaptera longimana of Europe, and Megaptera
          osphyia of America; that of the California coasts is
          Megaptera versabilis.
      (b) A small salmon (Oncorhynchus gorbuscha), of the
          northwest coast of America; the humpbacked salmon.
          [1913 Webster]

WordNet (r) 3.0 (2006):

humpback
    n 1: an abnormal backward curve to the vertebral column [syn:
         kyphosis, humpback, hunchback]
    2: a person whose back is hunched because of abnormal curvature
       of the upper spine [syn: humpback, hunchback,
       crookback]
    3: large whalebone whale with long flippers noted for arching or
       humping its back as it dives [syn: humpback, humpback
       whale, Megaptera novaeangliae]