infallible

adjective
  1. 1

    Without fault or weakness; incapable of error or fallacy.

    โ€œHe knows about many things, but even he is not infallible.โ€

  2. 2

    Certain to produce the intended effect, sure.

    โ€œTry this infallible cure for hiccups.โ€

Source: https://en.wiktionary.org/wiki/infallible