posthaste

noun
  1. 1

    A great hurry.

adverb
  1. 1

    Quickly, as fast as someone travelling post; with great speed

    โ€œIt is imperative that you finish your task posthaste.โ€

Synonyms

Source: https://en.wiktionary.org/wiki/post-haste, https://en.wiktionary.org/wiki/posthaste