The Collaborative International Dictionary of English v.0.48:

Posthaste \Post`haste\, adv.
   With speed or expedition; as, he traveled posthaste; to send
   posthaste. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Posthaste \Post`haste"\, n.
   Haste or speed in traveling, like that of a post or courier.
   --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

post-haste
    adv 1: as fast as possible; with all possible haste; "send it to
           me post-haste"

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "posthaste":
   amain, apace, at flank speed, breakneck, by airmail, by cable,
   by express, by return mail, by telegraph, double-quick,
   expeditious, expeditiously, express, fast, flat-out, fleet,
   full tilt, hand over fist, hand over hand, hastily, hasty,
   hell for leather, hell-bent, hell-bent for election,
   in double time, in double-quick time, in high, in high gear,
   in posthaste, in seven-league boots, lickety-cut, lickety-split,
   on the double, post, promptly, quick, quickly, rapid, rapidly,
   snappily, speedily, speedy, swift, swiftly, trippingly,
   under forced draft, whip and spur, with all haste,
   with giant strides, with rapid strides, with speed