to keep under lock and key
Seu segredo está guardado a sete chaves!
Your secret is kept under lock and key!
The image goes back to medieval Portugal: in the 13th century the royal court stored jewels and documents in chests fitted with four separate locks. The four keys were handed to four different officials, so the chest could only be opened when all of them were present, preventing theft or tampering.
Source: Só Português.
Falando's Idiom Trainer teaches you to recognize idiom meanings and create sentences using them, so you can speak like a Brazilian.
