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/ˌpəʊstˈheɪst/ · adverb
(formal, dated) Quickly, as fast as someone travelling post; with great speed..
It is imperative that you finish your task posthaste.
"Sasi cannot last through the night." These words from his physician, and the spectacle of my friend, now reduced almost to a skeleton, sent me posthaste to Serampore.
"And this, I take it, Is the main motive of our preparations The source of this our watch, and the chief head Of this post-haste and rummage in the land."
Use post haste when its meaning, tone and grammar fit the full sentence. A synonym is not always a direct replacement.
Post haste means (formal, dated) Quickly, as fast as someone travelling post; with great speed..
Common synonyms include ASAP, quickly.
The opposite depends on the specific sense.
It is imperative that you finish your task posthaste.
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