testament:  Testament is one of a range of English words that go back to Latin testis ‘witness’. This was derived from a prehistoric Indo-European base *tris- ‘three’, and so denoted etymologically a ‘third person’, who was not party to an agreement and thus could be a disinterested witness to it. Other English members of the testis family include testicle  (which etymologically ‘bears witness’ to a man’s virility), testify , testimony , and the prefixed forms attest , contest, detest, intestate , and protest.
The use of testament for ‘will’ was inspired by the notion of a ‘witnessed’ document. Its application to the two parts of the Bible arose from a mistranslation of Greek diathékē, which meant both ‘covenant’ and ‘will, testament’. It was used for the ‘covenant’ between God and human beings, but Latin translators rendered it as if it were being used for ‘will’ rather than ‘covenant’. => attest, contest, detest, intestate, protest, testicle, testify, testimony, three
late 13c., "last will disposing of property," from Latin testamentum "a last will, publication of a will," from testari "make a will, be witness to," from testis "witness," from PIE *tri-st-i- "third person standing by," from root *tris- "three" (see three) on the notion of "third person, disinterested witness."
Use in reference to the two divisions of the Bible (early 14c.) is from Late Latin vetus testamentum and novum testamentum, loan-translations of Greek palaia diatheke and kaine diatheke. Late Latin testamentum in this case was a confusion of the two meanings of Greek diatheke, which meant both "covenant, dispensation" and "will, testament," and was used in the former sense in the account of the Last Supper (see testimony) but subsequently was interpreted as Christ's "last will."