Where did the saying Merry Christmas come from?

Apparently 'Crimbo' is a slang synonym for the Word 'Christmas'.

The Word Christmas is a contraction of Christ's Mass, derived from the Old English Cristes mæsse and refering to the religious ceremony of mass. It is often abbreviated Xmas, probably because X or Xt have often been used as a contraction for Christ. The English letter X resembles the Greek letter (chi), the first letter of Christ in Greek transliterated as Christos. Crimbo is an informal synonym used in British English. Xmas is pronounced the same as Christmas, but most people just say X-Mas.