https://www.shopthesalvationarmy.com/Listing/Details/2645572...
They were intended to allow sailors to roll their pants up above the knee while swabbing the decks
https://www.wearethemighty.com/mighty-trending/this-is-why-t...
But we can see "bell bottomed trousers" referenced in popular culture in the 1940's, here's Louis Prima's band doing the song Bell Bottomed Trousers
https://www.youtube.com/watch?v=1QXmdbc5sx0
and here's a painting used for the sheet music
https://fineartamerica.com/featured/bell-bottom-trousers-alb...
https://www.ecrater.com/p/42407644/vintage-1944-bell-bottom-...