Definition of "dead giveaway" [dead giveaway]

• Something that discloses, usually unintentionally, a fact or an intention. (noun)

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• "Surely the masterminds behind this mission had understood that if the Empyreals came across a ship berth disguised as an asteroid it would be a dead giveaway that the Pax had slipped in a spy."
• "She was dying to check for a signal, but her eyescreens flicker would be a dead giveaway in the blackness of the tunnel."