from The Century Dictionary.
- Incapable of erring; infallible.
Sorry, no etymologies found.
But he adds a caution,  "That they are perfectly unalterable from their nature, and unerrable, if due circumstances be observed, that is, if due proposals be made to beget certain knowledge, and due care used to attend to such proposals."
 "It would require a large volume to unfold particularly how each virtue contributes to shew the unerrable indeficiency of tradition, and how the principles of almost each science are concerned in demonstrating its certainty: Arithmetic lends her numbering and multiplying faculty, to scan the vast number of testifiers; Geometry her proportions, to shew a kind of infinite strength of certitude in