post-haste adv.

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."

late 13c., "hurrying, haste; celerity, swiftness, speed;" c. 1300, "need for quick action, urgency;" from Old French haste "haste, urgency, hastiness" (12c., Modern French hâte), from Frankish *haifst "violence" or another Germanic source, from Proto-Germanic *haifstiz (source also of Gothic haifsts "strife," Old English hæste "violent, vehement, impetuous"). From late 14c. as "undue haste, rashness, unwise or unseemly quickness." To make haste "act quickly" is recorded by 1530s.

[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.