A petticoat is a piece of clothing like a thin skirt, which is worn under a skirt or dress.
英漢詞典釋義
n.
襯裙
I have to wear a petticoat under this skirt to prevent it from riding up when I move.我必須在裙子里面加一條襯裙,以防走動時裙子往上縮。That frock shows your petticoat.那件上衣太短,讓你的襯裙露出來了。