What does Letters Testamentary mean? And why is it not Letters OF Testamentary? This term may seem foreign to you, but it’s actually one of the more basic estate planning documents. As for the latter question, I really don’t know why it’s not Letters OF Testamentary. I guess attorneys just want to make things difficult.Letters Testamentary/Letters of Administration: What Is It?Letters Testamentary, similar to Letters of Administration, is a probate Court-issued document that authorizes a person to act as an Executor or Administrator of an estate. This Court Order gives the executor the authority to distribute the estate appropriately pursuant ...