if you know or feel something deep down, you are certain that it is true or you feel it strongly although you do not admit it or show it.

Deep down, she knew that what she was doing was wrong.

He tried to convince himself that he was enjoying his job, but deep down he was really miserable.

• cut two ways
• cut up rough
• cut your coat according to cloth
• cut your coat according to your cloth
• cut your losses
• cut your own throat
• cut your teeth
• cut your teeth in
• cut your teeth in something
• damned if do and damned if don't
• damned if do and damned if you don't
• damned if you do and damned if don't
• damned if you do and damned if you don't
• damn it
• damn someone with faint praise
• damn something with faint praise
• damn with faint praise
• damp squib
• dance to someone's tune
• dance to tune
• dance up a storm
• darkest hour is just before the dawn.
• dark horse
• dark-horse
• Davy Jones's locker