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1835, from French attaché "junior officer attached to the staff of an ambassador, etc.," literally "attached," past participle of attacher "to attach" (see attach). Attache case "small leather case for carrying papers" first recorded 1900.
A diplomatic officer attached to an embassy or consulate. Most attachés have specialties, such as military attachés, cultural attachés, economic attachés, and so forth.
Note: Some nations disguise spies as attachés.