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Webster 1913 Edition

Adj.

Infallible.

[Obs.]

“This unfailable word of truth.”

Bp. Hall.

Adj.

That cannot fail. [Not in use.]

unfailable ‎(comparative more unfailable, superlative most unfailable)

(obsolete) infallible
- Bishop Joseph Hall
  If then we believe this unfailable word of truth, who would not be content to mourn awhile, that he may rejoice for ever?