The name of a decree issued in Nazi Germany in December 1941, under which individuals suspected of resistance or other underground activities were arrested and deported suddenly and without trace, frequently during the night. In extended use: any situation, event, etc., characterized by mystery, obscurity, or secrecy.
1940s. From German Nacht und Nebel, lit. ‘night and fog’ from Nacht + und + Nebel mist, fog, haze.