'indefatigably' originates from Latin, specifically the word 'indefatigabilis,' where 'in-' meant 'not' and 'defatigare' meant 'to tire out.'

'indefatigabilis' transformed into the French word 'indefatigable,' and eventually became the modern English word 'indefatigable' through Middle English.

Initially, it meant 'not able to be tired out,' and this meaning has largely remained the same in modern usage.