A delegate cannot further delegate.
This Latin maxim means that a person to whom a power is delegated cannot further delegate that power. That person cannot sub-delegate that power to someone else. No sub-delegation can be done in a contract of principle-agent without the knowledge and consent of the principle. This principle is often used in administrative or constitutional law.
If a person has delegated ‘A’ to execute a will after his death, therefore, ‘A’ cannot further delegate this power to another person. If he does so, he will be delegating the power or authority, which was delegated to him.