What is the X in X-mas?
Old English lam "clay, mud, mire, earth," from Proto-Germanic *laimaz (cf. Old Saxon lemo, Dutch leem, German Lehm "loam"), from PIE root *(s)lei- "slimy" (see slime (n.)). As a type of highly fertile clayey soil, it is attested from 1660s. As a verb from c.1600.