What is the X in X-mas?
mid-14c., "alone, living alone," from Old French solitaire, from Latin solitarius "alone, lonely, isolated," from solitas "loneliness, solitude," from solus "alone" (see sole (adj.)). Meaning "single, sole, only" is from 1742. Related: Solitarily; solitariness. As a noun from late 14c.; from 1854 as short for solitary confinement (that phrase recorded from 1817).