fancy dress
noun [ U ] UK uk/ˌfæn.si ˈdres/ us/ˌfæn.si ˈdres/ (US costume)
化装舞会服裝
the special clothes that you wear for a party where everyone dresses as a particular type of character or thing
a fancy-dress party 化装舞会
I thought he was in fancy dress. 我以为他穿着化装舞会服装。