'defensibility' originates from Latin, specifically the verb 'defendere', where 'de-' meant 'away' and 'fendere' (from 'fendere'/'fend-') meant 'to strike or ward off'. The adjective element '-ible' comes via Latin/French meaning 'able to be', and the noun-forming suffix '-ity' comes from Latin '-itatem'.

'defendere' (Latin) led to Old French 'defendre' and Middle English 'defenden/defend'; the adjective 'defensible' (formed from Latin/French roots) combined with the noun suffix '-ity' in English to produce 'defensibility'.

Initially the root meant 'to ward off or repel (a blow)', but over time the derived forms came to mean 'capable of being defended' or 'capable of being justified', which is the modern sense of 'defensibility'.