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A "three-piece whistle" is a suit (Cockney rhyming slang - "whistle and flute" = "suit") comprising matching trousers (US: pants), waistcoat (US: vest) and jacket - more formal than a two-piece suit.
A "three-piece whistle" is a suit (Cockney rhyming slang - "whistle and flute" = "suit") comprising matching trousers (US: pants), waistcoat (US: vest) and jacket - more formal than a two-piece suit.