"Knickerbockers or knickers are a form of men´s or boys´ baggy-kneed trousers particularly popular in the early 20th-century United States. Golfers´ plus twos and plus fours ...
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are breeches of this type. Before World War II, skiers often wore knickerbockers too, usually ankle-length.
Until after World War I, in many English-speaking countries (excluding Britain), boys customarily wore short pants in summer and knickerbockers ("knickers" or "knee pants") in winter. At the onset of puberty, they graduated to long trousers. In that era, the transition to "long pants" was a major rite of passage. See, for example, the classic song "Blues in the Night" by Johnny Mercer: "My mammy done told me, when I was in knee-pants, my mammy done told me, son..."" - (en.wikipedia.org 11.09.2020)