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trousers, 1840, see pantaloons. Colloquial singular pant is attested from 1893. To wear the pants "be the dominant member of a household" is first attested 1931. To do something by the seat of (one's) pants "by human instinct" is from 1942, originally of pilots, perhaps with some notion of being able to sense the condition and situation of the plane by engine vibrations, etc. To be caught with (one's) pants down "discovered in an embarrassing condition" is from 1932.
ants, cream one's jeans, fancy pants, fly by the seat of one's pants, fudge one's pants, get the lead out, have lead in one's pants, high waters, hot pants, raggedy-ass, seat-of-the-pants, shit one's pants, smarty-pants
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