'uncatchable' originates from a combination of the negative prefix 'un-' (Old English), the verb 'catch' (from Middle English 'cachen', influenced by Old North French 'cachier'), and the adjective-forming suffix '-able' (from Latin '-abilis' via Old French).

'catch' changed from Middle English 'cachen' (itself influenced by Old North French 'cachier') and became the modern English 'catch'; the compound 'un-' + 'catch' + '-able' developed in Modern English to form 'uncatchable'.

Initially the components meant 'not' (un-), 'to seize or capture' (catch), and 'capable of' (-able); combined, they originally meant 'not able to be caught' and over time the word kept that basic sense while extending to figurative uses like 'difficult to defeat or grasp'.