Английский Этимологический словарь - testament

- c.1290, "last will disposing of property," from L. testamentum "a will, publication of a will," from testari "make a will, be witness to," from testis "witness," from PIE *tris- "three," on the notion of "third person, disinterested witness." Use in reference to the two divisions of the Bible (early 14c.) is from L.L. vetus testamentum and novum testamentum, loan-translations of Gk. palaia diatheke and kaine diatheke. L.L. testamentum was a mistranslation of Gk. diatheke, which meant both "covenant" and "will, testament," and was used in the former sense in the account of the Last Supper.