The Collaborative International Dictionary of English v.0.48:

Hornpipe \Horn"pipe`\, n. (Mus.)
   (a) An instrument of music formerly popular in Wales,
       consisting of a wooden pipe, with holes at intervals. It
       was so called because the bell at the open end was
       sometimes made of horn.
   (b) A lively tune played on a hornpipe, for dancing; a tune
       adapted for such playing.
       [1913 Webster]

             Many a hornpipe he tuned to his Phyllis. --Sir W.
                                                  Raleigh.
   (c) A dance performed, usually by one person, to such a tune,
       and popular among sailors.
       [1913 Webster]

WordNet (r) 3.0 (2006):

hornpipe
    n 1: a British solo dance performed by sailors
    2: music for dancing the hornpipe
    3: an ancient (now obsolete) single-reed woodwind; usually made
       of bone [syn: hornpipe, pibgorn, stockhorn]