Wiktionary
fat pants
n. (context plurale tantum slang English) Relatively large or stretchy pants (trousers) that are comfortable even when smaller pairs would feel tight.
Usage examples of "fat pants".
Catherine had fine, classic clothes in two sizes, made to fit her at about 145 and 165 pounds, Starling guessed, and there were a few pairs of crisis fat pants and pullovers from the Statuesque Shop.