The Collaborative International Dictionary of English v.0.48:

Connoisseur \Con`nois*seur"\ (?; 277), n. [F. connaisseur,
   formerly connoisseur, fr. conna[^i]tre to know, fr. L.
   cognoscere to become acquainted with; co- + noscere,
   gnoscere, to learn to know. See Know, and cf. Cognizor.]
   One well versed in any subject; a skillful or knowing person;
   a critical judge of any art, particulary of one of the fine
   arts.
   [1913 Webster]

         The connoisseur is "one who knows," as opposed to the
         dilettant, who only "thinks he knows."   --Fairholt.
   [1913 Webster]

WordNet (r) 3.0 (2006):

connoisseur
    n 1: an expert able to appreciate a field; especially in the
         fine arts [syn: connoisseur, cognoscente]

The Devil's Dictionary (1881-1906):

CONNOISSEUR, n.  A specialist who knows everything about something and
nothing about anything else.
    An old wine-bibber having been smashed in a railway collision,
some wine was pouted on his lips to revive him.  "Pauillac, 1873," he
murmured and died.