The story
From plastic + pants. Late 1940s, when polyvinyl chloride (a plastic) replaced latex as the principal material of the garment due to its lower cost. Compare earlier rubber pants.
Etymology from Wiktionary, CC BY-SA 4.0.
plastic pants
Today it means: An undergarment worn over a diaper or nappy (or underwear) in order to prevent leakage through the fabric. Full definition →