Also found in: Dictionary.
As quickly or promptly as possible. I need to get this application in the mail posthaste, or else my work permit is going to expire! Please report to your supervisor posthaste.
ˌpost-ˈhaste(literary) with great speed: I shall send the invitations off post-haste.This comes from the old phrase haste, post, haste, which was written on letters to tell the post (= the person taking the letters) to ride quickly to deliver them.
As quickly as possible. According to Ebenezer Brewer, in sixteenth-century England postal messengers galloping into an inn yard would cry “Post haste!” which gave them priority in selecting the horses available for their journey. Hall’s Chronicles of ca. 1548 stated, “The Duke of Somerset, with John, erle of Oxenford, wer in all post haste flying toward Scotlande.” Shakespeare used the expression in numerous plays, and despite its archaic sound and the dubious speed of present-day postal service, it remains current on both sides of the Atlantic. It is also written as one word, posthaste.