Ra-ra skirt
From wipipedia.org

A ra-ra skirt or rah-rah skirt is a loose miniskirt with layered frills. The name (first recorded in 1965) is said to derive from this type of skirt being worn by cheerleaders who shout "Ra! Ra!".
Being loose, a ra-ra skirt will flap around as the wearer walks, and energetic moves might allow a viewer an upskirt glimpse.