to keep quiet, to zip it
É melhor você ficar de bico calado sobre o que viu ontem à noite.
You better keep quiet about what you saw last night.
The image is transparent: bico literally means "beak," used colloquially for the mouth, so ficar de bico calado is to keep one's beak shut — to stay silent and let nothing slip.
Falando's Idiom Trainer teaches you to recognize idiom meanings and create sentences using them, so you can speak like a Brazilian.