What is the X in X-mas?
late 14c., "to trace back (to a first cause), attribute, assign," from Old French referer (14c.) and directly from Latin referre "to relate, refer," literally "to carry back," from re- "back" (see re-) + ferre "carry" (see infer). Meaning "to commit to some authority for a decision" is from mid-15c.; sense of "to direct (someone) to a book, etc." is from c.1600. Related: Referred; referring.