An archaic term meaning to fail, disappoint, or fall short of something; to become unfit or inadequate.
From Old French defaillir: de- (un-, not) + faillir (to fail, from Latin fallere). An old verb that has been largely replaced by 'fail' in modern English.
Defail is one of those archaic words that sounds like it should still exist—Old English had so many wonderful un- and de- verbs that we've lost through linguistic laziness.
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