I find it interesting that the word "costume" has taken on so narrow a meaning in the last 60+ years. As a lad, the term referred to any specialised clothing, as in "bring your swimming costume" or " students who do not own their own cricked costume, may borrow one from the school's stock".

I agree that it is largely (now) more or less of an insult to anyone wearing something out of the ordinary, ans all too often applied to the kilt.