What is the X in X-mas?
early 15c., "dispossess of appointed office," from Middle French desappointer (14c.) "undo the appointment, remove from office," from des- (see dis-) + appointer "appoint" (see appoint).
Modern sense of "to frustrate expectations" (late 15c.) is from secondary meaning of "fail to keep an appointment." Related: Disappointed; disappointing.