<P> The type known as "thermal underwear" is made from two - ply fabric of either a wool layer and an artificial fibre, only wool or--again mostly in the U.S.--two layers of only artificial fibres, which uses trapped body heat to insulate against cold air . </P> <P> The manufacturing foundations of long johns may lie in Derbyshire, England, at John Smedley's Lea Mills, located in Matlock . The company has a 225 - year heritage and is said to have created the garment, reputedly named after the late - 19th - century heavyweight boxer John L. Sullivan; the company still produces long johns . </P> <P> In 2004, Michael Quinion, a British etymologist and writer, postulated that the "john" in the item of apparel may be a reference to Sullivan, who wore a similar - looking garment in the ring . This explanation, however, is uncertain and the term's origin is ultimately unknown . So it is also very probable that it's simply a deformation of "longues jambes" meaning "long leg" in French . </P> <P> Long johns were first introduced into England in the 17th century, but they did not become popular as sleepwear until the 18th century . They were supposedly called long johns after a famous knife fighter who fought in long underwear . They were first used as loungewear but then later became popular as sleepwear . Children also wore long johns as sleepwear, usually made out of cotton and polyester . </P>

Where did the phrase pair of underwear come from