Resembling, characteristic of, or appearing like a costume; theatrical or artificial in appearance.
From 'costume' plus the adjective suffix '-y' (having the quality of). This informal adjective developed in modern English to describe something that looks staged or artificially dressed up.
When something looks 'costumey,' it means it's trying too hard to look a certain way—like if someone wore a full pirate outfit to the grocery store or a restaurant put fake English cottages everywhere. It suggests something is theatrical rather than authentic.
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