linguistic:paraphrasing
The word testament

The word testament in the expression "New Testament" refers to a Christian new covenant that Christians believe completes or fulfils the Mosaic covenant (the Jewish covenant) that Yahweh (the God of Israel) made with the people of Israel on Mount Sinai through Moses, described in the books of the Old Testament of the Christian Bible. While Christianity traditionally even claims this Christian new covenant as being prophesied in the Jewish Bible 's Book of Jeremiah, Judaism traditionally disagrees:

Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah;not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; forasmuch as they broke My covenant, although I was a lord over them, saith the LORD. But this is the covenant that I will make with the house of Israel after those days, saith the LORD, I will put My law in their inward parts, and in their heart will I write it; and I will be their God, and they shall be My people; and they shall teach no more every man his neighbour, and every man his brother, saying: 'Know the LORD'; for they shall all know Me, from the least of them unto the greatest of them, saith the LORD; for I will forgive their iniquity, and their sin will I remember no more.

The word covenant means 'agreement' (from Latin con-venio 'to agree' lit. 'to come together'): the use of the word testament, which describes the different idea of written instructions for inheritance after death, to refer to the covenant with Israel in the Old Testament, is foreign to the original Hebrew word brit (בְּרִית) describing it, which only means 'alliance, covenant, pact' and never 'inheritance instructions after death'. This use comes from the transcription of Latin testamentum 'will (left after death)', a literal translation of Greek diatheke (διαθήκη) 'will (left after death)', which is the word used to translate Hebrew brit in the Septuagint.

The choice of this word diatheke, by the Jewish translators of the Septuagint in Alexandria in the 3rd and 2nd century BCE, has been understood in Christian theology to imply a reinterpreted view of the Old Testament covenant with Israel as possessing characteristics of a 'will left after death' (the death of Jesus) and has generated considerable attention from biblical scholars and theologians: in contrast to the Jewish usage where brit was the usual Hebrew word used to refer to pacts, alliances and covenants in general, like a common pact between two individuals, and to the one between God and Israel in particular, in the Greek world diatheke was virtually never used to refer to an alliance or covenant (one exception is noted in a passage from Aristophanes) and referred instead to a will left after the death of a person. There is scholarly debate as to the reason why the translators of the Septuagint chose the term diatheke to translate Hebrew brit, instead of another Greek word generally used to refer to an alliance or covenant.