While searching for the correct spelling of inerrant, I came across a definition that labeled it as something that is unerring.
Why would something that is unerring be "inerrant" instead of "unerrant?"
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From what I could find, the word "unerring" came into use in 1640s from un-"not" + verbal noun from err (from Old French errer - "go astray, lose one's way; make a mistake; transgress").
The word "inerrant" came into use in 1650s, in reference to "fixed" stars (as opposed to "wandering" planets), from Latin inerrantem - "not wandering, fixed (of stars)", from in- "not, opposite of" and present participle of Latin errare ("to wander, stray, roam, rove"). Its usage as "unerring, free from error" began from 1785.
So, although the two words may be of the same origin (from Latin), they came into the English usage separately, each one by its own way.