Source : Webster's Revised Unabridged Dictionary (1913)

Incroyable \In`croy`a"ble\, n. [F., lit., incredible.]
   A French fop or dandy of the time of the Directory; hence,
   any fop.

   Note: The name is said to have been given in allusion not
         only to the extravagant dress, but also to the frequent
         use of the phrase ``C'est vraiment incroyable'' (That
         is really incredible.).