The Collaborative International Dictionary of English v.0.48:

Thoroughbred \Thor"ough*bred`\, a.
   Bred from the best blood through a long line; pure-blooded;
   -- said of stock, as horses. Hence, having the
   characteristics of such breeding; mettlesome; courageous; of
   elegant form, or the like. -- n. A thoroughbred animal,
   especially a horse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thoroughbred
    adj 1: having a list of ancestors as proof of being a purebred
           animal [syn: pedigree(a), pedigreed, pureblood,
           pureblooded, thoroughbred]
    n 1: a well-bred person
    2: a racehorse belonging to a breed that originated from a cross
       between Arabian stallions and English mares
    3: a pedigreed animal of unmixed lineage; used especially of
       horses [syn: thoroughbred, purebred, pureblood]