'indefinably' originates from the Latin word 'indefinibilis', where 'in-' meant 'not' and 'definibilis' meant 'able to be defined'.

'indefinibilis' transformed into the Old French word 'indefinable', and eventually became the modern English word 'indefinable', with 'indefinably' as its adverbial form.

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