fancy dress

plural fancy dresses

1. AustraliaUKuncountableusuallyA costume, disguise to masquerade as something or someone else.

   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

   1 more example

   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 Mephistopheles--this involving, as I need scarcely stress, not only scarlet tights but a pretty frightful false beard.

   1934, P. G. Wodehouse, Right Ho, Jeeves

2. uncountableusuallyFormalwear.
3. uncountableusuallyUsed other than figuratively or idiomatically: see fancy, dress.