Disciple is the root of Discipline.

Disciplining is the most difficult task for any manager.

Discipline is an important part of leadership. Groups count on their leaders to maintain standards of performance and fairness.

Avoid negative statements about the employee's character.
State facts and clarify expectations.
Allow an employee the option of meeting requirements or suffering consequences.
Maintain a business demeanor.

Showing anger is usually counter productive.
The boss who "loses it" may well have lost the ability to lead.