'non-testability' originates from Modern English, formed by the prefix 'non-' (meaning 'not') combined with the noun 'testability' (test + -ability).

'testability' is formed from the noun 'test' (Middle English from Old French/tester/ and related senses of examination or trial) plus the productive suffix '-ability' (from Latin '-abilitas' via Old French/Latin influence). The modern compound 'non-testability' is a later formation using the negative prefix 'non-' attached to 'testability'.

Originally 'testability' meant 'the quality of being able to be tested'; by prefixing with 'non-' the modern term came to mean 'the quality of not being able to be tested' or 'lack of testability'.