The bomb was built by the Metallurgical Laboratory of the University of Chicago, which had relocated to the University of Denver early in the fighting. The production of the Fat Lady was given a tremendous boost by a sample of plutonium which had been captured from the Race by a joint Soviet-German operation. The plutonium became the Fat Lady's core.
The bomb earned its nickname from its tremendous size. It weighed nearly 10 tons, so heavy that it could not be carried by airplane. The United States Army instead opted for railroad, transferring the Fat Lady successfully to Chicago, where it was used to wipe out a substantial number of Race infantrymales.
The "Fat Lady" is probably an allusion to "Fat Man", the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945, insofar as both used plutonium cores and both were "fat".