From Knitting Lingerie Style:



The origins of these mesh-sided leggings--they are inspired by the union suit, a type of one piece long underwear favored by men who work outside and for hunting, but also worn by women in cold climates.  Originally, in the 1800s, these garments were made of woven flannel, but were later fashioned of knit jersey.  Eventually the union suit was split up into separate tops and bottoms, and became known as long johns.  Such close-fitting underwear may have also inspired later outerwear fashions, such as the slim cigarette pants favored by women in the 1950s, the stirrup pants of the early 1960s, and the leggings of the 1980s. 



For these mesh-sided leggings, I chose an elasticized cotton yarn that assures a great fit (with no worries about baggy knees) and is cool enough to be comfortable, yet warm enough to be cozy.  The mesh stitch on the sides is a simple yarnover pattern, but it adds interest and lets a little flesh show though tastefully.