Wide knickerbockerbreeches, so called because originally they were designed with four inches of excess material so as to hang in a fold below the fastening beneath the knee;[1] traditionally associated with sports, especially golf.
Alan Gould. Every Creature of God Is Good. Publisher lulu.com 2009
And he looked quite smart in his brown plus fours. A few years ago he would not have dared to wear plus fours. They had belonged exclusively to the wealthier folk. Now everybody wore them. He was pleased with his pair. Quite cheap they had been too.
References
^ Brown, Lesley. The New shorter Oxford English dictionary on historical principles. Clarendon 1993 isbn=0-19-861271-0