The Collaborative International Dictionary of English v.0.48:

Haggis \Hag"gis\ (-g[i^]s), 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.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

haggis
    n 1: made of sheep's or calf's viscera minced with oatmeal and
         suet and onions and boiled in the animal's stomach