The phrase "one's medicine ball" is an idiomatic expression that means something very important or crucial to one's well-being, happiness, success, etc. It refers to a person's most cherished possession, goal, dream, ambition, or anything else that they hold dear and value highly. The phrase originated from the use of medicine balls in physical fitness training, where it is an essential tool for building strength, endurance, and coordination. In a similar way, one's "medicine ball" can be something that helps to build character, resilience or other positive qualities.