'infallible' originates from Medieval Latin, specifically the word 'infallibilis,' where 'in-' meant 'not' and 'fallibilis' meant 'liable to err.'

'infallibilis' transformed into the Old French word 'infallible,' and eventually became the modern English word 'infallible.'

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