1) Apace 2) At once 1) As soon as possible 2) Express mail 3) Immediately 4) Quickly 5) With great speed or immediacy 6) With utmost speed 
• (adv.) With speed or expedition; as, he traveled posthaste; to send posthaste. • (n.) Haste or speed in traveling, like that of a post or courier.
Post`haste' noun Haste or speed in traveling, like that of a post or courier. Shak. Post`haste adverb With speed or expedition; as, he traveled posthaste ; to send posthaste . Shak.