The term 'under lock and key' is a phrase used to describe something that is kept SAFE and SECURE. The crossword solutions for this phrase are 'SAFE' (4 letters) and 'SECURE' (6 letters). This phrase is often used to describe items or information that is kept in a SECURE place, such as a SAFE or vault, and is not accessible to the public. It can also be used to describe something that is kept confidential and not shared with anyone else.