Concise Bible Dictionary:

One who makes a will or testament, introduced in Hebrews 9:16-1716For where a testament is, there must also of necessity be the death of the testator. 17For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth. (Hebrews 9:16‑17) in a parenthesis, showing that as a will is of force only after a man is dead, so Christ must have died for the blessings of the new covenant to be available.