Trews (Gaelic Truis or older Truibhs) are men's clothing for the legs and lower abdomen, a traditional form of Scottish apparel. Trews could be trimmed with leather, probably buckskin, especially on the inner leg to prevent wear from riding on horseback.
Trews may be origin of the word trousers.
Tartan trews shared the fate of other items of Highland dress, including proscription under the Dress Act of 1746 that banned Highlanders from wearing the truis ("Trowse"),and rsurrection during the Romantic Revival; see kilt for a full discussion.
Illustrations in the Book of Kells and on the Cross of Muiredach show soldiers wearing short truis-like garments which reached to just above or just below the knee. Those illustrated in the Book of Kells are of a single colour, tight fitting and end below the knee while those shown on the cross panel are loose fitting, striped and gathered just above the knee. Also illustrated in the Book of Kells are long tight-fitting truis which are secured by loops under the foot.
The word is 'triubhas' in Scottish Gaelic. 'Truis' or 'trews' are anglicized spellings.
Edward Dwelly, Faclair Gàidhlig gu Beurla: Illustrated Gaelic to English Dictionary Colin Mark, The Gaelic-English Dictionary
Modern trews and Military trews
Military trews are usually worn by members of the lowland Scottish regiments as part of their dress uniforms and mess uniforms. Members of highland Scottish regiments are usually authorized to wear Kilts with these uniforms. Trews are also part of the uniform of the composite regiment known as The Highlanders (Seaforth, Gordons and Camerons).
Today, the sideways kicking step of Scottish highland dancers performing the Shean Truis dance is said to symbolise the kicking off of the trews or trousers in favour of the kilt.
Plus-fours and Golf Attire
Tartan Plus-fours are traditional golfing attire that are based on the Traditional trews. These pants extend 4 inches below the knee and are often worn with argyle knee-socks. Plus-fours were first introduced to the USA by the Prince of Wales during a 1924 visit. The full-length plaid pants popular with many golfers are also based on trews, but are cut fuller for more freedom of movement and warmer climates.