'indistinguishably' originates from the Latin word 'indistinguibilis', where 'in-' meant 'not' and 'distinguere' meant 'to separate or distinguish'.

'indistinguibilis' transformed into the Old French word 'indistinguible', and eventually became the modern English word 'indistinguishable', with 'indistinguishably' as its adverbial form.

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