mid-14c., "act of deception," from O.Fr. illusion "a mocking," from L. illusionem (nom. illusio) "a mocking, jesting, irony," from illudere "mock at," lit. "to play with," from in- "at" + ludere "to play" (see ludicrous). Sense of "deceptive appearance" developed in Eng. late 14c.
