'post-haste' originates from early modern English, formed from 'post' (from Old French/Latin related to the word 'post' meaning a mail/post or 'after') and 'haste' (from Old French 'haste' meaning speed or urgency).

'post-haste' appeared in late 16th to early 17th century English as the expression 'post haste' (often referring to speed as if sent by post/messenger) and later became commonly hyphenated as 'post-haste' and sometimes written 'posthaste.'

Initially it suggested speed associated with the post/messenger (swiftly by post); over time it generalized to mean 'immediately' or 'with great speed' in a wider sense.