Knickerbockers, later shortened to “
knickers”, were popular
casual wear for the well-dressed
gentleman in the 1920's. They were trousers with short legs that ended somewhere between the knee and the ankle.
Variations of knickers included plus-fours, plus-sixes, plus-eights and plus-tens. The “plus” in the term referred to how many inches below the knee they hung. With the knickers, gentlemen would wear Norfolk coats or golf coats. The coats sported large patch pockets, a belt, usually one button and often a shoulder yoke. Gentleman’s shoes or boots were the appropriate footwear to coordinate with knickerbockers.
Knickerbockers were forbidden in the classroom at Oxford University, which is why the Oxford bag was invented, a very baggy kind of trousers that could be worn over the knickers.
Source: http://www.murrayontravel.com/carolnolan/fashionhistory_1920mens.html