Xmas

Attested in this form since 1755, and preceded by earlier forms such as X'temmas (1551) and Old English Χp̃es mæssa (1100), from Cristes mæsse, with abbreviation of Cristes by the symbol X, orthographic borrowing from Ancient Greek Χ (Kh, letter chi), from Χριστός (Khristós, “Christ”). By surface analysis, X (“Christ”) +‎ -mas (“holiday”). In popular use since late 19th century. See Wikipedia for more information.