If you ask someone to do something Post Haste you are want them to do it quickly, without delay. The early English postal system of the 1700s relied on horseback messengers. Horses would need to be rested every 20 to 30 miles and this led to posthouses springing up all over the country - all providing fresh horses for messengers to use on longer journeys. The royal post was always prioritised and post-boys would gallop into stable yards shouting "post haste" to attract attention and then swap their horse for a rested one to carry on his journey. This system also led to the expression By Return Post, meaning immediate reply. A messenger asked to bring a reply to a letter back on his return journey would wait until the recipient provided it before going.