Her curly hair bounced as she stomped back and forth, her soft, round buttocks flexing under her long-johns.
type of warm underwear, 1943, originally for U.S. GIs. By 1919 as a type of pastry. Long john also was used of various sorts of worm, potato, sled, etc.
Long winter underwear: A generation or so ago, men, women, boys and girls put on long ones in October and wore long ones until spring/ Pop, will you dig that dish out of your long johns (1940s+)