What is the X in X-mas?
word-forming element meaning "two," Old English twi-, prefix meaning "two, in two ways, twice, double," from Proto-Germanic *twi- (cf. Old Frisian twi-, Old Norse tvi-, Dutch twee-, Old High German zwi-, German zwei-), from PIE *dwis (cf. Sanskrit dvi-, Greek di-, Old Latin dvi-, Latin bi-, Lithuanian dvi-), from *dwo "two" (see two). Cognate with bi-. Older instances of it include Middle English twinter "two years old" (c.1400, of cattle, sheep, etc.), reduced from Old English twi-wintre, and Old English twispræc "double or deceitful speech."