Fat Man was the United States codename for the atomic bomb dropped on Nagasaki before Japan's surrender.
The plutonium weapon was detonated over Nagasaki on 9 August 1945, three days after Little Boy was used against Hiroshima. A B-29 bomber named Bockscar carried Fat Man, and the explosion devastated large areas of the city. Japan's leadership faced this bombing alongside the Soviet declaration of war and invasion of Manchuria.
The bomb's name was part of a set of informal wartime labels. Little Boy referred to the uranium bomb used at Hiroshima. Thin Man was an earlier design concept that was abandoned, while Gadget was the name of the plutonium device tested at Trinity in July 1945.
Japan announced its acceptance of the Allied surrender terms on 15 August. The formal surrender document was signed on 2 September aboard USS Missouri, so Fat Man's use preceded the legal conclusion of the war.