a serious hidden danger; a major threat from within or close at the core of something

(idiom) a serious trouble or danger hidden within; a threat close at hand; also used figuratively to mean a fatal disease

a secret or personal matter kept deep in one's heart that is not shared easily with others

A fatal disease or illness within the body; often used to describe a serious internal threat or a hidden danger close at hand.

a very close, intimate, and trusted friend

private words or heartfelt secrets shared only with very close, trusted friends