'unintelligibly' originates from the Latin word 'intelligibilis,' where 'intelligere' meant 'to understand.' The prefix 'un-' negates the meaning, indicating a lack of understanding.

'intelligibilis' transformed into the Old French word 'intelligible,' and eventually became the modern English word 'intelligible,' with 'unintelligibly' as its adverbial form.

Initially, it meant 'capable of being understood,' but with the prefix 'un-,' it evolved into its current meaning of 'impossible to understand.'