The phrase "stock still" means to be completely still, motionless, or immobile. The antonyms of "stock still" are words that mean movement, activity, or motion. These antonyms include words like active, lively, dynamic, animated, mobile, and agile. These words describe a state of being that is opposite to being still, static or motionless. For instance, an active person is always moving and engaging in various physical activities, while a lively atmosphere is full of energy and movement. Therefore, if you are looking to describe something that is in motion or full of energy, you should use these antonyms instead of "stock still.