What is the X in X-mas?
late 13c., from Anglo-French eleccioun, Old French elecion "choice, election, selection" (12c.), from Latin electionem (nominative electio), noun of action from past participle stem of eligere "pick out, select," from ex- "out" (see ex-) + -ligere, comb. form of legere "to choose, read" (see lecture (n.)). Theological sense is from late 14c.