NORDO, short for "No Radio", is a North American aviation term for aircraft flying without a radio. The term may originate from the 5-character uppercase abbreviated notation "NORDO" displayed on controllers' radar scopes when an aircraft transmits the "radio failure" code on its transponder. An alternate explanation is that "NO RDO" was the standard note made on maintenance and equipment sheets used in military aviation, starting in the 1930s, as a code to identify planes which needed radio repairs or were not equipped with radios.
Another little Americanism to creep into the vernacular..had to google it to be sure.