A garment becomes seated when the fabric fibers permanently yield to the tension of the wearer's body in a sitting position. This deformation leaves a baggy, sagging pouch in the rear seat area even when the wearer stands up. It most commonly affects tightly fitted skirts and trousers made from natural fibers lacking elastane recovery.
Heard on the job
That pencil skirt is completely seated, so you'll need to steam and block it back into shape before the fitting.