nemesisEnglishOxford American Thesaurusnemesis noun 1 this could prove to be the bank's nemesis: downfall, undoing, ruin, ruination, destruction, Waterloo. 2 will Harry Potter finally defeat his nemesis, Voldemort?: archenemy, arch-rival, enemy, rival, foe, adversary, opponent, antagonist, combatant, challenger. 3 the nemesis that his crime deserved: retribution, vengeance, retributive justice, punishment, just deserts; fate, destiny.