posthaste: Meaning and Definition of

post•haste

Pronunciation: (pōst'hāst'), [key]
— adv.
  1. with the greatest possible speed or promptness: to come to a friend's aid posthaste.
—n.
  1. great haste.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.