infallible

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.”

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.”

   “In precisely the same way does a quack doctor prescribe his infallible nostrum to every patient, without taking into account differences of constitution, or [...]”