Wiktionary
n. A kind of woman's lingerie resembling long shorts, usually with ruffles down each leg.
Wikipedia
Pettipants are a type of lingerie worn by women. The name is a portmanteau of petticoat (ultimately from French petit, "small") and pants. Pettipants are similar to long shorts, though they may be made from material such as cotton and lace, and usually have ruffles down each leg. They are available in different lengths up to knee length.
When pettipants were fashionable, they were usually worn under skirts, dresses, culottes, or walking shorts for modesty or comfort. However, they are not considered a modern or popular style; currently they are most likely to be worn by square dancers or persons involved in historical reenactment.
Unlike other types of underwear, pettipants will not ride up and eliminate hot-weather chafing.