"a timber of considerable size set upright," from Old English post "pillar, doorpost," and from Old French post "post, upright beam," both from Latin postis "door, post, doorpost," in Medieval Latin "a beam, rod, pole," which is perhaps from Vulgar Latin *por- "forth," a variant of pro- (see pro-) + stare "to stand" (from PIE root *sta- "to stand, make or be firm").
Similar compounds are Sanskrit prstham "back, roof, peak," Avestan parshti "back," Greek pastas "porch in front of a house, colonnade," Middle High German virst "ridgepole," Lithuanian pirštas, Old Church Slavonic pristu "finger" (PIE *por-st-i-).
Later also of metal. As a type of hardness, lifelessness, deafness by early 15c.
"station when on duty, a fixed position or place," 1590s, from 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; the meaning "job, position, position" is attested by 1690s. The military meaning "fort, permanent quarters for troops" is by 1703.
[mail system] c. 1500, "riders and horses posted at intervals," to provide direct and rapid communication of messages and letters from one place to another by relays, from post (n.2) on notion of riders and horses "posted" at intervals along a route. Probably formed on model of French poste in this sense (late 15c.).
The meaning "system for the conveyance of letters" is from 1660s; it is attested from 1590s in the sense of "vehicle used to convey mails;" 1670s as "a dispatch of letters from or to a place." As a newspaper name from 1680s.
"to affix (a paper notice, advertisement, etc.) to a post" (in a public place), hence, "to make known, to bring before the public," 1630s, from post (n.1). The meaning "to achieve" (a score, a victory) appears to have begin in U.S. newspaper sports-writing, by 1949. Related: Posted; posting.
"to send through the postal system," 1837, from post (n.3). Earlier, "to travel with relays of horses" (1530s), hence "to ride rapidly" (1560s). Related: Posted; posting.
"to station at a place," 1680s, from post (n.2) "place when on duty." Related: Posted; posting.
1540s, "with post horses," hence, "rapidly;" especially in the phrase to ride post "go rapidly," from post (n.3) "riders and horses posted at intervals."