The Origin of 'Posthaste'
If you didn't already know the etymology of
posthaste, you might see the
post at the beginning of the word and assume that it's functioning as a prefix meaning "after," the way it does in Latin words like
postmortem, or in English words like
postgame or
postgraduate, or in movements of art or critical theory like
postmodernism or
post-structuralism.
Not quite. The
post in
posthaste has to do with the mail. In Middle English,
post haste was a noun for the speed with which a person delivering mail was pressed to do their job.
In the 16th century, "haste, post, haste" was used to inform couriers (also called
posts) that a letter was urgent.
Post-haste later came to mean great promptness and speed for any purpose, and was used in phrases like
in post-haste and
in all post-haste.
In other words, the work of a courier was so routinely associated with speed and efficiency that it was used as a reference point in the language for others doing speedy labors. The notion caught on so quickly that
post-haste was seeing use as an adjective and adverb by the end of the 16th century.
https://www.merriam-webster.com/words-at-play/posthaste-word-history