What is the X in X-mas?
c.1300, "to own, possess, enjoy the possession of, have the fruition of," from Old French rejoiss-, present participle stem of rejoir, resjoir "gladden, rejoice," from re-, which here is of obscure signification, perhaps an intensive (see re-), + joir "be glad," from Latin gaudere "rejoice" (see joy).
Originally sense in to rejoice in. Meaning "to be full of joy" is recorded from late 14c. Middle English also used simple verb joy "to feel gladness; to rejoice" (mid-13c.) and rejoy (early 14c.). Related: Rejoiced; rejoicing.