different between infallible vs unfailable

English

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

infallible (comparative more infallible, superlative most infallible)

Without fault or weakness; incapable of error or fallacy.
He knows about many things, but even he is not infallible.
Certain to produce the intended effect, sure.
Try this infallible cure for hiccups.

un- +? fail +? -able

unfailable (comparative more unfailable, superlative most unfailable)

(obsolete) infallible
- c. 1652, Joseph Hall, The Mourner in Sion (sermon)
  If then we believe this unfailable word of truth, who would not be content to mourn awhile, that he may rejoice for ever?

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