What is the X in X-mas?
c.1300, "matching, similar, correponding," present participle adjective and adverb from accord (v.). Meanings "conforming (to), compliant, in agreement; consistent, harmonious; suitable, appropriate" are from late 14c. According to "referring to," literally "in a manner agreeing with" is attested from mid-15c.
late 13c., accourd, from Old French acord "agreement," a back-formation from acorder (see accord (v.)).