Fig: to find out about a secret or an information that was kept hidden

Slot machines! That's what you do every night when you disappear for an hour after work. Well, well, the cat's out of the bag!

Slot machines! Ecco che cosa fai ogni sera quando sparisci per un'ora dopo il lavoro. Bene, bene, si scoprono gli altarini!

Brenda, a close friend of my wife, saw me leaving Lara's hotel room early this morning. I'm afraid the cat's out of the bag now!

Brenda, un'amica intima di mia moglie, mi ha visto uscire questa mattina presto dalla camera d'albergo di Lara. Ho paura che ormai si siano scoperti gli altarini!

In older days, con-men at county fairs would put a cat in a “poke” (a small sack) 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”