posthaste means quickly, as fast as someone travelling post; with great speed.
posthaste is pronounced /ˌpəʊstˈheɪst/.
Why “posthaste” is a great word
With the greatest possible speed, conveying the urgency of a dispatched courier. The word originates from the instruction “haste, post, haste” scrawled upon urgent missives, later compacted into a single term marrying ‘post,’ the relay system of messengers, with ‘haste,’ first recorded in the 1530s. Unlike acting “expeditiously,” which implies systematic, efficient dispatch, or moving “gradually,” its direct antonym, “posthaste” emphasizes raw, unbridled immediacy. It is the pounding of hooves on a muddy road after midnight, the frantic rifling through saddlebags for a sealed packet, the breathless arrival at a gate just as it is closing—a word that captures the desperate, physical strain against the creeping shadow of delay.
Etymology
From the former instruction on letters ‘haste, post, haste’, later reinterpreted as a compound of post + haste.
adv
- Quickly, as fast as someone travelling post; with great speed.e.g.“It is imperative that you finish your task posthaste.”