What is the X in X-mas?
early 14c., from Old French austere (Modern French austère) and directly from Latin austerus "dry, harsh, sour, tart," from Greek austeros "bitter, harsh," especially "making the tongue dry" (originally used of fruits, wines), metaphorically "austere, harsh," from PIE *saus- "dry" (cf. Greek auos "dry," auein "to dry"). Use in English is figurative: "stern, severe, very simple." Related: Austerely.