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]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "croup":
   Asiatic flu, Hong Kong flu, acute bronchitis, adenoiditis,
   aluminosis, amygdalitis, anthracosilicosis, anthracosis,
   asbestosis, asthma, atypical pneumonia, backside, behind,
   bituminosis, black lung, bronchial pneumonia, bronchiectasis,
   bronchiolitis, bronchitis, bronchopneumonia, butt, buttocks,
   catarrh, chalicosis, chronic bronchitis, cold, collapsed lung,
   common cold, coniosis, coryza, croupous pneumonia, crupper,
   double pneumonia, dry pleurisy, emphysema, empyema,
   epidemic pleurodynia, fibrinous pneumonia, flu, grippe, haunches,
   hay fever, hind end, influenza, la grippe, laryngitis,
   lipoid pneumonia, lobar pneumonia, lung cancer, lung fever, nates,
   pharyngitis, pleurisy, pleuritis, pneumococcal pneumonia,
   pneumoconiosis, pneumonia, pneumonic fever, pneumothorax,
   posterior, quinsy, rear, rear end, rheum, rump, siderosis,
   silicosis, sore throat, swine flu, the sniffles, the snuffles,
   tonsilitis, virus pneumonia, wet pleurisy, whooping cough