/ Dictionary / Index F Fat Man: Meaning and Definition of Find definitions for: Fat' Man" Pronunciation: [key] the code name for the plutonium-core, implosion-type atom bomb the U.S. first tested and then dropped on Nagasaki in 1945. Cf. Little Boy. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. fatly fat meat Related Content Daily Word Quiz: philtrum Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language