[ReneeLynn]

I thought ‘fancy dress’ meant costumes as in for a costume party.

[Wolfie]

Indeed, “Fancy Dress” is British for costume, worn to a “Fancy Dress Party”.

Two countries seperated by a common language, and all that.

[OpeEdMunkey]

I thought ‘fancy dress’ meant costumes as in for a costume party.

You are correct. That makes it even funnier.