What is the X in X-mas?
Old English paþ, pæþ "path, track," from West Germanic *patha- (cf. Old Frisian path, Middle Dutch pat, Dutch pad, Old High German pfad, German Pfad "path"), of unknown origin. The original initial -p- in a Germanic word is an etymological puzzle. Watkins says the word is "probably borrowed (? via Scythian) from Iranian *path-," from PIE root *pent- "to tread, go, pass" (cf. Avestan patha "way;" see find (v.)), but this is too much of a stretch for OED and others. In Scotland and Northern England, commonly a steep ascent of a hill or in a road.