let the cat out of the bag reveal a secret, especially carelessly or by mistake.

☞ A similar metaphorical use of bag may be found in the French phrase vider le sac, literally 'empty the bag', meaning 'tell the whole story'.

1996Bernard ConnollyThe Rotten Heart of EuropeTim Renton…at odds with his leader on Europe, let the cat out of the bag when he told a television audience, 'we need a strong Europe to maintain our independence from the United States and the Pacific Rim'.