Fig.: To give away a secret. To reveal information or facts previously hidden that shouldn't be shared

I wanted to keep it under wraps for a while, but as you let the cat out of the bag, I'm going to have to announce that I've taken another job

Volevo tenerlo segreto per un po', ma dato che te lo sei lasciato sfuggire, mi trovo costretto ad ammettere che ho accettato un altro posto di lavoro

We wanted to keep our wedding plans under wraps to surprise my parents, but my sister let the cat out of the bag and… there went the surprise

Volevamo tenere segreti i nostri progetti matrimoniali per fare una sorpresa ai miei, ma mia sorella se l'è lasciato sfuggire e… addio sorpresa!

A journalist said on TV that he found the perfect little village for peace and tranquillity. The villagers should find the free publicity cheering but may not be glad that someone let the cat out of the bag

Un giornalista ha dichiarato alla TV di aver trovato un paesino perfetto per pace e tranquillità. Gli abitanti dovrebbero rallegrarsi della pubblicità gratuita, ma potrebbero anche non essere contenti che qualcuno abbia rivelato il loro segreto

In older days, con-men at county fairs would put a cat in a small sack or a bag and, claiming it was a suckling piglet, find someone who was simple enough to buy the "pig" without even looking into the bag, because everyone knew how hard it would be to catch the piglet if it got loose. A canny purchaser, however, might refuse to buy without an examination of the contents, and, by opening the sack, would literally "let the cat out of the bag"