Here's why:
* Drill: To drill can mean to practice something repeatedly, often in a military context. It can also refer to the act of making a hole with a drill.
* Train: To train means to teach someone a skill or prepare them for a specific task.
* Discipline: This word encompasses both meanings. It refers to the strict training that involves repeated practice and imposes order and control, which can be applied to the body (physical discipline) or the mind (mental discipline).
Therefore, discipline accurately describes the act of both drilling and training the body.