What is the X in X-mas?
early 14c., "evening signal, ringing of a bell at a fixed hour," from Anglo-French coeverfu (late 13c.), from Old French cuevrefeu, literally "cover fire" (Modern French couvre-few), from cuevre, imperative of covrir "to cover" (see cover (v.)) + feu "fire" (see focus (n.)). The medieval practice of ringing a bell at fixed time in the evening as an order to bank the hearths and prepare for sleep. The original purpose was to prevent conflagrations from untended fires. The modern extended sense of "periodic restriction of movement" had evolved by 1800s.