infallible

Etymology

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

noun

1. A person who, or an object or process that, is taken as being infallible.

adj

1. Without fault or weakness; incapable of error or fallacy.
   He knows about many things, but even he is not infallible.
   That there may be Certainty upon an infallible Evidence in Matters of Science, I readily grant you. But since there once were Scepticks in Philosophy as well as Religion, such as doubted of every thing, I very much question, whether the whole World be agreed in this Point; unless you could assure me, that Race of Seekers is now extinct. 1720, Samuel Fancourt, “The Remarker's second Objection produced and examined”, in An Essay Concerning Certainty and Infallibility: Or, Some Reflections Upon a Pamphlet Stiled, “The Nature and Consequences of Enthusiasm Considered, in Some Short Remarks on the Doctrine of the Blessed Trinity Stated and Defended.” In a Letter to the Author of Those Remarks, London: R. Cruttenden, page 35
2. Certain to produce the intended effect; sure.
   Try this infallible cure for hiccups.
   [L]isten patiently … 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. 1818, Mary Shelley, chapter 4, in Frankenstein, archived from the original on 2011-10-30
   In precisely the same way does a quack doctor prescribe his infallible nostrum to every patient, without taking into account differences of constitution, or … 1833, James Rennie, “The Word Gardening”, in Alphabet of Scientific Gardening for the Use of Beginners, London: William Orr, page 2