'incontestably' originates from Late Latin 'incontestabilis', where 'in-' meant 'not' and the root related to 'contestare/contestabilis' meant 'able to be contested or disputed'.

'incontestabilis' passed into Old French as 'incontestable' and then into Middle English/early Modern English as 'incontestable', from which the adverb 'incontestably' was formed by adding the adverbial suffix '-ly'.

Initially it meant 'not able to be contested or disputed', and over time it has retained that sense, coming to mean 'unquestionably' or 'beyond dispute' in modern usage.