The Collaborative International Dictionary of English v.0.48:

Croup \Croup\ (kr[=oo]p), n. [F. croupe hind quarters, croup,
   rump, of German or Icel. origin; cf. Icel. kryppa hump; akin
   to Icel. kroppr. Cf. Crop.]
   The hinder part or buttocks of certain quadrupeds, especially
   of a horse; hence, the place behind the saddle.
   [1913 Webster]

         So light to the croup the fair lady he swung,
         So light to the saddle before her he sprung. --Sir W.
                                                  Scott.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Croup \Croup\ (kr[=oo]p), n. [Scot. croup, cf. croup, crowp, to
   croak, to cry or speak with a hoarse voice; cf. also LG.
   kropp, G. kropf, the crop or craw of a bird, and tumor on the
   anterior part of the neck, a wen, etc. Cf. Crop.] (Med.)
   An inflammatory affection of the larynx or trachea,
   accompanied by a hoarse, ringing cough and stridulous,
   difficult breathing; esp., such an affection when associated
   with the development of a false membrane in the air passages
   (also called membranous croup). See False croup, under
   False, and Diphtheria.
   [1913 Webster]

WordNet (r) 3.0 (2006):

croup
    n 1: a disease of infants and young children; harsh coughing and
         hoarseness and fever and difficult breathing [syn: croup,
         spasmodic laryngitis]
    2: the part of an animal that corresponds to the human buttocks
       [syn: hindquarters, croup, croupe, rump]