Synonyms for Incontestable (same or very similar meaning)
WordNet sense 1 (not open to question; obviously true):
indisputable, undisputableWordNet sense 2 (not possible to deny):
undeniableWordNet sense 3 (necessarily or demonstrably true):
incontrovertibleWordNet sense 4 (incapable of being contested or disputed):
incontestibleFrom the ODE community, based on WordNetadd/edit