A person who makes or provides costumes, especially for theatrical productions; a costumer.
From French 'costumier' (one who deals in costumes), derived from 'costume'. This is a more formal or sophisticated variant of the English 'costumer', particularly used in British and theatrical contexts.
The French spelling 'costumier' sounds more elegant and is exactly the kind of word borrowed into English to describe something fashionable or theatrical. It's the same way we say 'café' instead of 'coffee house'—borrowed words feel more sophisticated.
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