Pertaining to a will or testament: as testamentary causes. Derived from, founded on, or appointed by a testament or will: as a testamentary guardian, letters testamentary, etc. A paper, instrument, document, gift, appointment. etc., is said to be “testamentary” when it is written or made so as not to take effect until after the death of the person making it, and to be revocable and retain the property under his control during his life, although he may have believed that it would operate as an instrument of a different character. Sweet.