The Collaborative International Dictionary of English v.0.48:

Fin de siecle \Fin` de si[`e]"cle\ [F.]
   Lit., end of the century; -- mostly used adjectively in
   English to signify: belonging to, or characteristic of, the
   close of the 19th century. At that time the phrase was also
   intended to imply "modern" or "up-to-date;" as,
   fin-de-si[`e]cle ideas.

   Note: As the twentieth century comes to a close, the meaning
         may change.
         [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

fin de siecle
    adj 1: relating to or characteristic of the end of a century
           (especially the end of the 19th century); "fin de siecle
           art"