An archaic term for failure, faltering, or the act of falling short; a weakness or lack of success.
From Old French defaillance derived from defaillir (to fail). This noun form has largely been replaced by 'failure' or 'failance' in modern English.
Medieval French had the beautiful word 'defaillance' to describe that moment of weakness when you're about to give up—we lost a whole vocabulary of failure when we switched languages.
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