Longman Dictionary of Contemporary English
fancy dress
noun
EXAMPLES FROM CORPUS
▪ A masked fancy dress ball in Cannes, especially at the Villa Russe, was an event.
▪ A mini ramp jam with fancy dress and the odd one foot tail grab ollie.
▪ Did you hear about the man who went to the fancy dress party as a bone?
▪ Every day the children worry about their Hallowe'en costumes for the school fancy dress parade.
▪ However, hand on heart, even if you are a top dressage rider, can you really justify the fancy dress?
▪ In their fancy dress, they ordered pizza before going by motorcade to the Tennessee ball.
▪ Less serious runners also compete wearing fancy dress.
▪ No, I am not going to a fancy dress party, but to work.
Wiktionary
fancy dress
n. 1 (context UK English) A costume, disguise to masquerade as something or someone else. 2 (context uncountable English) formalwear.
WordNet
fancy dress
n. a costume worn as a disguise at a masquerade party [syn: masquerade, masquerade costume]
Wikipedia
Fancy Dress (film)
Fancy Dress is a 1919 British silent comedy film directed by Kenelm Foss and starring Godfrey Tearle, Ivy Duke and Guy Newall. A lawyer hires a strolling player to impersonate an aristocrat.
Usage examples of "fancy dress".
But the men who commanded the black troops stood out not only because of the color of their skin but also for their fancy dress.