What is the X in X-mas?
1650s, "cold, icy," from French glacial, from Latin glacialis "icy, frozen, full of ice," from glacies "ice," probably from PIE root *gel- "cold" (cf. Latin gelu "frost;" see cold (adj.)). Geological sense apparently coined in 1846 by British naturalist Edward Forbes (1815-1854). Related: Glacially.