'incontestability' originates from the adjective 'incontestable' + the noun-forming suffix '-ity'; 'incontestable' itself comes from Latin/Medieval Latin elements where the prefix 'in-' meant 'not' and the root related to 'contestare'/'testari' (to call to witness, to dispute).

'contestare' (Latin, to call to witness / to dispute) developed into Old French/Latin forms such as 'contester/contestabilis'; Middle English adopted related forms (e.g. 'contesten', 'contestable'), leading to the adjective 'incontestable' in Early Modern English and eventually to the noun 'incontestability' by adding '-ity'.

Initially derived forms referred to 'not able to be called into question (not to be contested)'; over time this evolved into the modern meaning emphasizing 'indisputability' or 'the state of being beyond dispute'.