no decir ni pío

to keep mum; to not breathe a word; to not say a word; to not breathe a word

“–Has hecho lo que debías. Pero si te hablan de él, tú ni pío. Es posible incluso que lo veas en el periódico. Pero tú no digas ni pío.” — "You've said you had to. But if they talk to you about it, not a word. You may well even see it in the papers. But don't you breathe a word."

Literally, “to not say 'chirp'”.