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Trousers : Originally a pair of loose cloth tubes worn by medieval farm workers to protect their legs. Held up by tying at the upper leg. They obviously came in pairs, hence trousers is a true plural.
Pantaloons : Popular in Tudor England and Europe. A garment for the lower body from the waist to about the knee. Would have originally be worn as an undergarment while wearing armour. As fashionable wear, they became very baggy and elaborate. This is where the American word "pants" comes from.
Britches : Came into fashion around 17th century (maybe earlier). Still survive today. A garment for the lower body extending from the waist and (separately) down the legs to about the knee. They are fastened at the knee. Much closer fitting than pantaloons.
Sometime around the 18th Century modern style trousers appeared. worn like britches, but with full length, loose fitting legs (like the original trousers). This is where the modern English word trouser comes from.
Knickers : In American, a form of baggy, loose fitting britches. In English... female underpants in the 19th century, worn under huge skirts and petticoats, looked very similar to the American knickers (worn by men). It is a common use slang term. Bloomers is another name. It has continued in use for the modern equivalent female underwear.
Can I have a "Best Answer" please?