Knickerbockers, or knickers in the United States (US), are a form of baggy-kneed breeches, particularly popular in the early 20th-century United States. Golfers' plus twos and plus fours are similar. Until after World War I, in many English-speaking countries, boys customarily wore short pants in summer and "knee pants" similar to knickers in winter. At the onset of puberty or sometime in their teens, they graduated to long trousers. In that era, the transition to "long pants" was a major rite of passage. Men continued to wear knickerbockers for athletics, outdoor work, and other informal activities for which they were practical. During the early 20th century, knickerbockers were also increasingly worn by women.

Source: Wikipedia