While the wearing of trousers by women took solid root in WWII, the practice dates back nearly a century. The first women to dress one leg at a time on a regular basis were the romanticized mine workers known as The Wigan Pit Brow Lasses. Their outfit of choice included skirts worn over trousers, with the skirts rolled up at the waist to keep the hems out of the way when working.
"Pit brow women working outside in the cold and dirt developed a distinctive 'uniform', they wore clogs, trousers covered with a skirt and apron, old flannel jackets or shawls and headscarfs to protect their hair from coal dust. The women's unconventional but practical dress drew them to the attention of the public and carte de visite and cabinet card portraits and later postcards of them in working clothes were produced commercially and sold to visitors as novelties." [source: Wikipedia]
"Before 1842 it was common for women to work underground at the pits, usually as drawers pulling carts of coal through tunnels up to the surface. Lancashire in fact had a much larger number of women working in the mines than elsewhere in Britain. The Mines Act of 1842 however made it illegal for women to work underground. The law was generally ignored, with women dressing in men’s clothes and bosses turning a blind eye. Women could earn 2 or 3 shillings a week more if they worked underground and the tasks above ground could be just as hard work and dangerous, but for less money." [source: lightshaweadows.co.uk]
Revolutions in Fashion
The sporting life for women, popularized by cultural icon Coco Chanel, saw the rise of leg coverings for women as a practical part of engaging in athletics. The way had already been paved in the 1890s with the rise in cycling. Turkish style bloomers became part and parcel of the cycling costume, and Parisian women took to wearing them both for fashion and exercise. Bloomers were then introduced as apparel for basketball, swimming, and eventually as 'gym class uniforms'.