'indefensibly' originates from the Latin word 'indefensibilis', where 'in-' meant 'not' and 'defensibilis' meant 'capable of being defended'.

'indefensibilis' transformed into the Old French word 'indefensible', and eventually became the modern English word 'indefensible', with 'indefensibly' as its adverbial form.

Initially, it meant 'not capable of being defended', and this meaning has largely remained the same in modern usage.