"Fat Man" was the codename for the atomic bomb dropped on Nagasaki, Japan, on August 9, 1945, during World War II. It was a plutonium-based bomb, designed to create a powerful explosion through nuclear fission. The bomb weighed about 10,300 pounds and had a yield equivalent to approximately 21 kilotons of TNT. The development of "Fat Man" was part of the Manhattan Project, a secret U.S. government research initiative aimed at building atomic weapons. The bombing of Nagasaki, along with the earlier bombing of Hiroshima by the Little Boy bomb, played a significant role in Japan's surrender and the end of the war.