spill the beans

Spill the beans - revelar um segredo, deixar escapar um segredo de propósito

"It was then that she threatened to spill the beans about her affair with the president."
(www.thefreedictionary.com)

let the cat out of the bag

let the cat out of the bag: revelar um segredo

"Please, don’t let the cat out of the bag! She'll be mad at you!"