noun, masculine/kɔ.ti.jɔ̃/plural: cotillons
petticoat; cotillion, cotillon; party novelties (e.g. paper hats, streamers etc.)
“À trois heures du matin, le cotillon commença. Emma ne savait pas valser.”
From cotte (“dress”) + -illon (“diminutive”) + (alt. -ille + -on).