"Let the cat out of the bag" means-

• "Let the cat out of the bag" means - Reveal a secret carelessly.

• Let the cat out of the bag (idiom)
- English Meaning: to allow a secret to be known, usually without intending to; reveal a secret carelessly or by mistake.
- Bangla Meaning: হাটে হাঁড়ি ভাঙা/গোপন কথা বলে ফেলা।

Examples:
- I was trying to keep the party a secret, but Mel went and let the cat out of the bag.
- He let the cat out of the bag, and the surprise party wasn't a surprise after all
- Some idiot's let the cat out of the bag -- Mrs Salman realizes there's something going on.

Source:
1. Cambridge Dictionary.
2. Oxford Dictionary.