What is the X in X-mas?
The English language from about 1150 to about 1500. During this time, following the Norman Conquest of England, the native language of England — Old English — borrowed great numbers of words from the Norman French of the conquerors. Middle English eventually developed into modern English.
Note: Many of the writings in Middle English that have survived have word forms very different from those in modern English; today's readers of English cannot understand the language of these works without training. Some dialects of Middle English, however, resemble modern English, and a good reader of today can catch the drift of something written in them. Geoffrey Chaucer wrote The Canterbury Tales in one of these dialects.