1. (n.) Haste or speed in traveling, like that of a post or courier.

2. (adv.) With speed or expedition; as, he traveled posthaste; to send 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