What is the X in X-mas?
1590s, plural of Latin illuminatus "enlightened" (in figurative sense), past participle of illuminare (see illumination). Originally applied to a 16c. Spanish sect (the Alumbrados), then to other sects; since 1797 used as a translation of German Illuminaten, name of a secret society founded 1776 in Ingolstadt, Bavaria, and holding deistic and republican principles; hence used generally of free-thinkers and sarcastically of those professing intellectual enlightenment (1816). Related: Illuminatism; illuminatist.