Acqua in bocca

Literally: Water in the mouth

Example: “Ti dico del regalo, ma acqua in bocca!” — I'll tell you about the present, but mum's the word!

English equivalent: Mum's the word

Origin: According to Florentine lexicographer Pirro Giacchi's 1878 Dizionario del vernacolo fiorentino, a devout but notoriously gossiping and sharp-tongued woman confessed her habit of speaking ill of others and asked her priest for help resisting the temptation. He gave her a small bottle of well water and told her to place a few drops in her mouth every time she felt the urge to gossip rising — a discreet, practical trick that worked simply because it's physically impossible to speak clearly with your mouth full of water. No firm historical documentation confirms this specific story, but the image has an obvious, universal logic that needed little further explanation to spread: keeping water in your mouth is a simple, effective way of keeping your mouth shut about anything you've been asked to keep quiet.