illusion: [14] The notion of ‘play’ is at the etymological heart of illusion (as indeed of its close relatives allusion [16], delusion [15], and elude [16]). It came via Old French from Latin illūsiō, a derivative of illūdere ‘make fun of’. This was a compound verb formed from the prefix in- and lūdere ‘play’ (source of English ludicrous [17]). In classical Latin illūsiō meant ‘mockery’, and no semantic shift seems to have taken place until post-classical times, when it moved to ‘deceit’ (a sense originally taken over by English). => allusion, delusion, elude, ludicrous
illusion (n.)
mid-14c., "act of deception," from Old French illusion "a mocking, deceit, deception" (12c.), from Latin illusionem (nominative illusio) "a mocking, jesting, irony," from illudere "mock at," literally "to play with," from assimilated form of in- "at, upon" (see in- (2)) + ludere "to play" (see ludicrous). Sense of "deceptive appearance" developed in Church Latin and was attested in English by late 14c. Related: Illusioned "full of illusions" (1920).