Some opaque plastic pants
Plastic pants, plastic panties, and diaper covers are all basically the same thing. They are devices worn over a diaper for the purpose of containing liquid or solid waste that may otherwise leak through the fabric. Today, "plastic pants" are usually made out of polyvinyl chloride (PVC) or polyurethane (PU) though, in some instances, the traditional latex rubber is still used. The original diaper covers were made from oiled wool, which was naturally waterproof.
Plastic panties are a fetish for some. The enjoy the feel of plastic and the rush they make when worn under normal attire when in public places. Plastic panties when made to wear for a (bath)brush spanking increase the discomfort because of the sweat developing inside across the targeted bottom.