Thank my wise-ass grandfather for this one.

Xmas (also X-mas) is a common abbreviation of the word Christmas. It is sometimes pronounced /ˈɛksməs/, but Xmas, and variants such as Xtemass, originated as handwriting abbreviations for the typical pronunciation /ˈkrɪsməs/. The "X" comes from the Greek letter Chi, which is the first letter of the Greek word Christós (Greek: Χριστός), which became Christ in English.[1] The suffix -mas is from the Latin-derived Old English word for Mass.[2]