Определение и Значение "(as|) still as a stone"

What is the origin of the idiom "still as a stone" and when to use it?

The origin of the idiom "still as a stone" lies in the observation that stones or rocks typically remain motionless and unaffected by external factors. The phrase is used to describe someone or something that is completely still, calm, and unchanging, much like an unmoving stone. It conveys a sense of tranquility, stability, and lack of movement or disturbance.