Chapter Positiveness to Post of P by Webster's Dictionary (1913 Edition)

1. To travel with post horses; figuratively, to travel in haste. "Post seedily to my lord your husband." Shak.

And post o'er land and ocean without rest.

Milton.

2. (Man.) To rise and sink in the saddle, in accordance with the motion of the horse, esp. in trotting. [Eng.]

Post
(Post), adv. With post horses; hence, in haste; as, to travel post.