Posthaste - Idioms by The Free Dictionary
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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.