'unfailing' originates from Middle English, specifically the word 'failen,' where 'un-' meant 'not' and 'failen' meant 'to fail.'

'failen' transformed into the modern English word 'fail,' and with the prefix 'un-' it became 'unfailing,' eventually leading to the adverb 'unfailingly.'

Initially, it meant 'not failing,' and this meaning has largely remained the same in modern usage.