The Collaborative International Dictionary of English v.0.48:

Confidant \Con`fi*dant"\; 277), n. masc., Confidante
\Con`fi*dante"\ (?; 277), n. fem.[F. confident, confidente,
   formerly also spelt confidant, confidante. See Confide, and
   cf. Confident.]
   One to whom secrets, especially those relating to affairs of
   love, are confided or intrusted; a confidential or bosom
   friend.
   [1913 Webster]

         You love me for no other end
         Than to become my confidant and friend;
         As such I keep no secret from your sight. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

confidant
    n 1: someone to whom private matters are confided [syn:
         confidant, intimate]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "confidant":
   Dutch uncle, Polonius, acquaintance, admonisher, adviser, advocate,
   alter ego, amigo, backer, backseat driver, best friend,
   bosom friend, brother, buttinsky, casual acquaintance,
   close acquaintance, close friend, confidante, consultant, counsel,
   counselor, expert, familiar, favorer, fellow, fellow creature,
   fellowman, fidus Achates, friend, guide, inseparable friend,
   instructor, intimate, kibitzer, lover, man Friday, mate, meddler,
   mentor, monitor, neighbor, nestor, orienter, other self, partisan,
   pickup, preceptist, repository, right hand, right-hand man,
   second self, strong right hand, supporter, sympathizer, teacher,
   well-wisher



The Devil's Dictionary (1881-1906):

CONFIDANT, CONFIDANTE, n.  One entrusted by A with the secrets of B,
confided by _him_ to C.