What is the X in X-mas?
"a timber set upright," from Old English post "pillar, doorpost," and Old French post "post, upright beam," both from Latin postis "door, post, doorpost," perhaps from por- "forth" (see pro-) + stare "to stand" (see stet). Similar compound in Sanskrit prstham "back, roof, peak," Avestan parshti "back," Greek pastas "porch in front of a house, colonnade," Middle High German virst "ridepole," Lithuanian pirstas, Old Church Slavonic pristu "finger" (PIE *por-st-i-).
"place when on duty," 1590s, from Middle French poste "place where one is stationed," also, "station for post horses" (16c.), from Italian posto "post, station," from Vulgar Latin *postum, from Latin positum, neuter past participle of ponere "to place, to put" (see position (n.)). Earliest sense in English was military; meaning "job, position" is attested 1690s.
"mail system," c.1500, "riders and horses posted at intervals," from post (n.2) on notion of riders and horses "posted" at intervals along a route to speed mail in relays, probably formed on model of Middle French poste in this sense (late 15c.). Meaning "system for carrying mail" is from 1660s.
"to affix (a paper, etc.) to a post" (in a public place), hence, "to make known," 1630s, from post (n.1). Related: Posted; posting.
"to send through the postal system," 1837, from post (n.3). Earlier, "to travel with relays of horses" (1530s). Related: Posted; posting.
"to put up bail money," 1781, from one of the nouns post, but which one is uncertain. Related: Posted; posting.
"to station at a post," from post (n.2). Related: Posted; posting.
1540s, "with post horses," hence, "rapidly;" especially in the phrase to ride post "go rapidly," from post (n.3).
(1.) A runner, or courier, for the rapid transmission of letters, etc. (2 Chr. 30:6; Esther 3:13, 15; 8:10, 14; Job 9:25; Jer. 51:31). Such messengers were used from very early times. Those employed by the Hebrew kings had a military character (1 Sam. 22:17; 2 Kings 10:25, "guard," marg. "runners"). The modern system of postal communication was first established by Louis XI. of France in A.D. 1464. (2.) This word sometimes also is used for lintel or threshold (Isa. 6:4).