Sir Isaac Newton did not invent the newton meter. Although the identity of the inventor remains unknown, it is unlikely that Newton invented it because the newton was defined and named in the 19th century, 60 years after his death in 1727.
The newton meter is a device that measures torque, or energy required to cause rotation. It is derived from the product of the force and distance between the rotation's center and the point where force has been applied. Torque is a different physical concept than energy or work, so the term "newton" is applied to torque rather than "joules." One newton meter is equal to 0.737562149 foot pounds.