fancy dress

Noun

fancy dress (usually uncountable, plural fancy dresses)

1. (Australia, UK, usually) A costume, disguise to masquerade as something or someone else.
   • And yet here he was, if one could credit one's senses, about to take part in a fancy-dress ball, a form of entertainment notoriously a testing experience for the toughest. And he was attending that fancy-dress ball, mark you--not, like every other well-bred Englishman, as a Pierrot, but as...1934, P. G. Wodehouse, Right Ho, Jeeves
   • Royal Mail worker Evette Chapman gathered a team of 12 colleagues to deliver post in fancy dress and raise money for a nurses' charity and patients in Musgrove Park Hospital, Taunton.2020 November 18, “Stop & Examine”, in Rail, page 71
2. (usually) Formalwear.
3. (usually) Used other than figuratively or idiomatically: see fancy, dress.