infallible
Pronunciation:
/ɪnˈfa.lɪ.b(ə)l//ɪnˈfæ.lə.bəl/adjective
Definitions
Without fault or weakness; incapable of error or fallacy.
Example: He knows about many things, but even he is not infallible.
Certain to produce the intended effect, sure.
Example: Try this infallible cure for hiccups.