What is the X in X-mas?
Old English purpul, dissimilation (first recorded in Northumbrian, in Lindisfarne gospel) of purpure "purple dye, a purple garment," purpuren (adj.) "purple," a borrowing by 9c. from Latin purpura "purple color, purple-dyed cloak, purple dye," also "shellfish from which purple was made," and "splendid attire generally," from Greek porphyra "purple dye, purple" (cf. porphyry), of uncertain origin, perhaps Semitic, originally the name for the shellfish (murex) from which it was obtained. Purpur continued as a parallel form until 15c., and through 19c. in heraldry. As a color name, attested from early 15c. Tyrian purple, produced around Tyre, was prized as dye for royal garments.
Also the color of mourning or penitence (especially in royalty or clergy). Rhetorical for "splendid, gaudy" (of prose) from 1590s. Purple Heart, U.S. decoration for service members wounded in combat, instituted 1932; originally a cloth decoration begun by George Washington in 1782. Hendrix' Purple Haze (1967) is slang for "LSD."
c.1400, from purple (n.). Related: Purpled; purpling.