'inefficacy' originates from Latin, specifically the word 'inefficacia,' where 'in-' meant 'not' and 'efficacia' meant 'efficacy.'

'inefficacia' transformed into the French word 'inefficacité,' and eventually became the modern English word 'inefficacy' through Middle English.

Initially, it meant 'lack of efficacy,' and this meaning has largely remained the same in modern usage.