posthaste I. \(ˈ) ̷ ̷| ̷ ̷\noun Etymology:post (IV)(courier) + haste archaic: speed in traveling (as of a post or courier) : great haste — used chiefly in the phrase in posthaste II. adverb : with great speed : in great haste : by the fastest possible means < riding posthaste > < sent posthaste for his lawyer > III. adjective obsolete: speedy, immediate < requires your haste, posthaste appearance — Shakespeare >