That means suicide mission.

How the Crew of U-672 Survived Their Suicide Mission — In July 1944, U-672 received the suicidal order to attack Allied ships at the mouth of the Seine. A destroyer damaged the submarine, but its crew managed to escape. The wreck has now been discovered in the English Channel…

On July 18, U-672 was spotted by the British frigate HMS “Balfour” and attacked with Hedgehog launchers loaded with 24 grenades. The submarine sprang a leak, but the crew managed to keep it submerged until nightfall. Lawaetz then gave the order to surface and abandon the submarine. He and the engineering officer were the last to leave the submarine, after destroying the Enigma cipher machine and opening the flood valves. Drifting in rubber dinghies, the crew was picked up by Allied ships. All 52 crew members were taken alive to a British prisoner-of-war camp.