What is the X in X-mas?
"secret, hidden" (obsolete), from Old English derne "concealed, secret, dark," from West Germanic *darnjaz (cf. Old Saxon derni, Old Frisian dern, Old High German tarni "secret, concealed").
As a verb, "to conceal," from Old English diernan "to hide." Cf. Old High German tarnjan "to conceal, hide;" German Tarnkappe "cloak of invisibility." Related to dark (adj.). French ternir "to tarnish, to dull" is a Germanic loan-word.