What is the X in X-mas?
mid-14c., "civil or military official," from Old French prefect (12c., Modern French préfet) and directly from Latin praefectus "public overseer, superintendent, director," noun use of past participle of praeficere "to put in front, to set over, put in authority," from prae "in front, before" (see pre-) + root of facere (past participle factus) "to perform" (see factitious). Spelling restored from Middle English prefet. Meaning "administrative head of the Paris police" is from 1800; meaning "senior pupil designated to keep order in an English school" is from 1864. Related: Prefectorial.