Wikipedia
Ban-Lon (sometimes spelled BanLon or Banlon) is a trademarked, multistrand, continuous-filament synthetic yarn used in the retail clothing industry. It is artificially crimped in order to achieve greater bulk than ordinary yarns. It is frequently associated with 1950s and 1960s American clothing and culture, and at the time was most commonly used to make sweaters.
Ban-Lon came to be used as a punchline for jokes in films and on television shows in the 1990s. In an episode of NBC's Seinfeld series, Frank Costanza said that because of his "man breasts" he "wouldn't be caught dead in Ban-Lon", and in the film Romy and Michele's High School Reunion, Michelle comments, "...how am I gonna impress anybody by selling Ban-Lon smocks at Bargain Mart?"