What does “minha boca é um túmulo” mean in Portuguese?

Minha boca é um túmulo literally means “my mouth is a tomb” — and it’s how Brazilians promise total secrecy, the way English says “my lips are sealed.” A túmulo is a grave, so the image is that whatever you tell me is buried for good: nothing gets out. It’s everyday, slightly playful, and exactly what you say when someone trusts you with a secret.

The whole phrase is the unit: minha boca (“my mouth”) + é um túmulo (“is a tomb”). The same grave imagery powers a sister expression, levar o segredo para o túmulo (“take the secret to the grave”). Here a teacher uses it as a deadpan joke while handing out an exam — “minha boca é um túmulo agora, ó. Não sei de nada” (“my lips are sealed now — I don’t know a thing”) — pretending she has no idea what’s on the test. That mock-innocent twist is a very natural way the idiom shows up in real speech.