Source : Webster's Revised Unabridged Dictionary (1913)

Haggis \Hag"gis\, n. [Scot. hag to hack, chop, E. hack. Formed,
   perhaps, in imitation of the F. hachis (E. hash), fr.
   hacher.]
   A Scotch pudding made of the heart, liver, lights, etc., of a
   sheep or lamb, minced with suet, onions, oatmeal, etc.,
   highly seasoned, and boiled in the stomach of the same
   animal; minced head and pluck. [Written also {haggiss},
   {haggess}, and {haggies}.]

Source : WordNet®

haggis
     n : made of sheep's or calf's viscera minced with oatmeal and
         suet and onions and boiled in the animal's stomach