This is the noun: the quality of not being talked out of a fight. If someone shows implacability, no “sorry,” no gift, and no kind words will make them stop opposing you. The anger, the hunt, or the political fight does not cool down. It is a heavy, rather formal word; you meet it in news, history, and novels more than at the dinner table. Though actually, English speakers use the adjective implacable far more than this noun. People reach for it when they want to say the opposition itself will not melt.