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1610s, earlier trouzes (1580s), extended from trouse (1570s), with plural ending typical of things in pairs, from Gaelic or Middle Irish triubhas "close-fitting shorts," of uncertain origin. The unexplained intrusive second -r- is perhaps by influence of drawers.
an outer garment covering the lower half of the body from the waist to the ankles and divided into sections to cover each leg separately. In attempting to define trousers, historians often explain that if any portion of a garment passed between the legs, it was an ancestor of the trousers. Thus defined, trousers can be traced to ancient times as worn, for example, by the Scythians, Persians, Japanese, and Hindus