plus fours 
plural noun
Baggy knickerbockers reaching below the knee, formerly worn by men for hunting and golf: he was dressed for an Edwardian shooting party in a full suit of tweed plus fours...
- His Royal Highness, the Prince of Wales, arrived by train overnight, pulled on his plus fours and a beret, grabbed his shooting stick and headed for Carnoustie for the final day of the 1931 Open.
- Since then he has exhibited worldwide - at overseas exhibitions he plays his Englishness to the hilt by turning up wearing plus fours and a deerstalker.
- Several male refugees packed plus fours in the belief that they constituted an essential part of a respectable Englishman's wardrobe.
Origin
1920s: so named because the overhang at the knee required an extra four inches of material.