Definition of Fat man

"Fat Man" is the codename, allegedly named after Winston Churchill, of the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the 2nd of the two nuclear weapons to be used in warfare and was the third man-made nuclear explosion of all time. The name also refers more generically to the early nuclear weapon designs of U.S. weapons based on the "Fat Man" model. It was an implosion-type weapon with a plutonium core.