'unapprehensible' originates from the prefix 'un-' meaning 'not' and the word 'apprehensible' which comes from Latin 'apprehensibilis', where 'apprehendere' meant 'to seize or grasp'.

'apprehensibilis' transformed into the Old French word 'apprehensible', and eventually became the modern English word 'apprehensible', with 'un-' added to form 'unapprehensible'.

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