Webster's 1828 Dictionary
POST-HASTE
n.Haste or speed in traveling, like that of a post or courier.
POST-HASTE
adv. With speed or expedition. He traveled post-haste, that is, by an ellipsis, with post-haste.
Oxford Dictionary
post-haste
post-haste ▶adverb with great speed or immediacy: she would go post-haste to England. ORIGIN mid 16th cent.: from the direction ‘haste, post, haste ’, formerly given on letters.
Oxford Thesaurus
post-haste
post-haste adverb he departed post-haste for Venice: as quickly as possible, without delay, (very ) quickly, speedily, swiftly, without further /more ado, with all speed, promptly, immediately, at once, straight away, right away, directly, forthwith; informal double quick, p.d.q. (pretty damn quick ), pronto, before you can say Jack Robinson, straight off; archaic straight, instanter.
Sanseido Wisdom Dictionary
post-haste
p ò st-h á ste 副詞 ｟古 ｠大急ぎで .
The Wordsmith Dictionary