incroyable (n.)

1796, from French incroyable, literally "incredible" (15c.), from in- "not, opposite of, without" (see in- (1)) + croire "to believe," from Latin credere "to believe" (see credo). A name for the French fop or dandy of the period of the Directory (1795-1799). Said to be so called from their extravagant dress, and also, according to OED, from a favorite expression among them ("C'est vraiment incroyable").