top secret
/tɒp ˈsiːkrət
/adjective
Of the highest secrecy; highly confidential: the experiments were top secret...
- He handled - and secretly photographed - top-secret military documents that crossed his desk.
- The real reason he is never free to visit you isn't because the CIA forbids him from leaving his top-secret mission in Uruguay.
- I knew they couldn't because a few days before their president had provided me with a top-secret document from the computer.