What is the X in X-mas?
early 15c., "intellectual, talented," from Middle French ingénieux "clever, ingenious" (Old French engeignos), from Latin ingeniosus "of good capacity, full of intellect; clever, gifted with genius," from ingenium "innate qualities, ability," literally "that which is inborn," from in- "in" (see in- (2)) + gignere, from PIE *gen- "produce" (see genus). Sense of "skillful, clever at contrivance" first recorded 1540s. In a sense of "crafty, clever, skillful" Middle English had enginous (mid-14c.), from Old French engeignos. Related: Ingeniously; ingeniousness.