infallible

noun
1

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

adjective
1

Without fault or weakness; incapable of error or fallacy.

"He knows about many things, but even he is not infallible."

2

Certain to produce the intended effect; sure.

"Try this infallible cure for hiccups."

3

Certain to result from a specific cause; inevitable.

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

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