A petticoat is a piece of clothing like a thin skirt, which is worn under a skirt or dress.
英汉词典释义
n.
衬裙
I have to wear a petticoat under this skirt to prevent it from riding up when I move.我必须在裙子里面加一条衬裙,以防走动时裙子往上缩。That frock shows your petticoat.那件上衣太短,让你的衬裙露出来了。