What is the X in X-mas?
late 14c., "infinitely short space of time," from Old French instant (adj.) "assiduous, at hand," from Medieval Latin instantem (nominative instans), in classical Latin "present, pressing, urgent," literally "standing near," present participle of instare "to urge, to stand near, be present (to urge one's case)," from in- "in" (see in- (2)) + stare "to stand," from PIE root *sta- "to stand" (see stet). Elliptical use of the French adjective as a noun.
mid-15c., "present, urgent," from Old French instant (14c.), from Latin instantem (nominative instans) "pressing, urgent," literally "standing near" (see instant (n.)). Meaning "now, present" is from 1540s, and led to the use of the word in dating of correspondence, in reference to the current month, often abbreviated inst. and persisting at least into the mid-19c. Thus 16th inst. means "sixteenth of the current month." Sense of "immediately" is from 1590s. Of foods, by 1912. Televised sports instant replay attested by 1965. Instant messaging attested by 1994.