What is the X in X-mas?
Old English heofonlic "celestial; chaste;" see heaven + -ly (1). Meaning "beautiful, divinely lovely" is late 14c., often (though not originally) with reference to the celestial "music of the spheres;" weakened sense of "excellent, enjoyable" is first recorded 1874. The heavenly bodies (stars, planets, etc.) attested from late 14c. Related: Heavenliness.