At rest and Stock-still

At rest adjective - In a state of repose or especially sleep.

Stock-still and at rest are semantically related in calm topic. In some cases you can use "Stock-still" instead an adjective phrase "At rest", when it comes to topics like motionless.

Stock-still adjective - Absolutely still.

At rest and stock-still are semantically related in motionless topic. You can use "At rest" instead an adjective phrase "Stock-still".