British informal word for a swimsuit or bathing costume.
Shortened form of 'costume' or 'bathing costume,' common in British English from early 20th century, following the pattern of informal British abbreviations ending in -ie.
This British slang shows how casual speech shortens words—'cossie' might seem random until you realize it's just 'costume' with the first part clipped off, the same way Americans say 'tux' for tuxedo.
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