'unapprehendable' originates from English, specifically the prefix 'un-' and the adjective 'apprehendable', where 'un-' meant 'not' and 'apprehendable' ultimately comes from Latin 'apprehendere' meaning 'to seize, grasp'.

'apprehend' comes from Latin 'apprehendere' (ad- 'to' + prehendere 'to grasp'), passed into Old French and Middle English as forms such as 'apprehenden' and 'apprehenden', later becoming 'apprehend' in modern English; the adjective-forming suffix '-able' produced 'apprehendable', and the negative prefix 'un-' was added in English to form 'unapprehendable'.

Initially 'apprehendere' meant 'to seize or take hold of' physically; over time the sense extended to 'to grasp mentally' (understand), and 'unapprehendable' now carries the meaning 'not capable of being grasped or understood' (or, in literal contexts, 'not capable of being captured').