What is the X in X-mas?
mid-15c. (grammatical, also as a noun); 1550s in general use, from Middle French possessif (15c.) "relating to possession, possessive," and directly from Latin possessivus, from possess-, past participle stem of possidere "to possess" (see possess). Related: Possessively; possessiveness.
The case of a noun or pronoun that shows possession. Nouns are usually made possessive by adding an apostrophe and s: “The bicycle is Sue's, not Mark's.” Possessive pronouns can take the place of possessive nouns: “The bicycle is hers, not his.” (See nominative case and objective case.)