Infallible

From Medieval Latin infallibilis, from Latin in- + fallibilis. Compare French infaillible.

Full definition of infallible

Without fault or weakness; incapable of error or fallacy.He knows about many things, but even he is not infallible.
certain, sure.

1818, Mary Shelley, Frankenstein Chapter 4, I see by your eagerness and the wonder and hope which your eyes express, my friend, that you expect to be informed of the secret with which I am acquainted; that cannot be; listen patiently until the end of my story, and you will easily perceive why I am reserved upon that subject. I will not lead you on, unguarded and ardent as I then was, to your destruction and infallible misery.