Duhaime's Law Dictionary Quash Definition: To set aside. To set aside; to rescind; to vacate; to render null and void. To quash a conviction means to set it aside as if it never existed. A motion to quash seeks to rescind a previous decision or a document or decision. If a subpoena is issued improperly, a motion might be presented to the Court to quash the subpoena. A certiorari application may request of the Court that the lower tribunal's decision be quashed. Always looking up definitions? Save time with our search provider (modern browsers only) If you find an error or omission in Duhaime's Law Dictionary, or if you have suggestion for a legal term, we'd love to hear from you!