Fat is the customary, everyday word applying to someone who has too much flabby tissue: "Is it true that nobody loves a fat man?" Whereas fat usually has as unpleasant connotation, plump and stout suggest a roundness that is pleasing or a heavy build that is not unpleasant: "This is a jolly plump (or stout) old man." "Her stout (or plump) figure was attractively covered by a well-designed gown." Obese, meaning "excessively fat," "overweight," is used more often in scientific circles than by the general public: "The physician outlined a strict diet for all his obese patients." If none of these terms expresses what you have in mind, consider corpulent, well-fed, adipose, pudgy, portly, bulky, thickset, rotund, chubby, and fleshy.
Fat appears in many slangy or trite expressions for which less-worn synonyms can usually be found: "fat chance" ("slight chance"); "fat lot" ("little" or "not at all"); "chew the fat" ("engage in informal conversation"); "the fat is in the fire" ("the action is started and cannot be stopped"); "the fat of the land" ("the best of anything"); "fat cat" ("wealthy or important person"); "fathead" ("stupid person"); "fats," "fatso" ("overweight person").
Dictionary of problem words and expressions. Harry Shaw. 1975.