"Fat Man" was the code name given to the atomic bomb dropped over Japan by the United States of America near the end of World War II.
After Jonathan Archer restored a damaged timeline, an image of "Fat Man" could be seen in the time stream as the timeline realigned itself. (ENT: "Storm Front, Part II")
An image depicting "Fat Man" was scanned by the Talosians as they reviewed the library computer files on board USS Enterprise in 2254. (TOS: "The Cage" remastered)