As I clearly move in the wrong circles, this was a revelation to me. ‘Sponge-bag trousers’ is the name given, in both American and Etonian slang (I am proficient in neither), to the striped trousers worn with morning coats and similar formal daywear for gentlemen.
The name apparently derives from the pattern of the cloth, of random width stripes, which was also traditionally used in sponge-bags. This stripe is ‘Cashmere stripe’ and has nothing to do with cashmere, nor with pin-stripes which are equidistant in width and separation.
Thank you Wikipedia for the edification and the image of the upright, sponge-bag be-trousered man.
‘A portly gentleman wearing a frock-coat and sponge-bag trousers.’
Source: Virginia Woolf, Orlando: A Biography, ed. with an introduction by Rachel Bowlby (Oxford: World’s Classics, 1992), p. 222