Last night, Clinton and Sanders sparred over the question of what was or was not possible for an executive branch faced with a majority-opposition Congress. But many Americans watching the debate were stuck on a different question: what does implacable mean?
In a discussion about the environment during the Democratic debate, Bernie Sanders told Hillary Clinton that the recent Paris climate change agreement was not a concrete accomplishment: “We have to get beyond paper right now.”
In response to Bernie Sanders's statement that the recent Paris climate change agreement was not a concrete accomplishment—“We have to get beyond paper right now”—Hillary Clinton argued that the agreement creates
...the framework to actually take the action that would have only come about because under the Obama administration in the face of implacable hostility from the Republicans in Congress President Obama moved forward on gas mileage, he moved forward on the clean power plant.
Implacable means “opposed to someone or something in a very angry or determined way that cannot be changed.”
Implacable comes to English from the French word of the same spelling; the French took it from the Latin implacabilis ("unappeasable, irreconcilable"). It shares its roots with placate, and has been in our language since the 15th century.
Implacable may also be used in reference to something that is inexorable, as when Vladimir Nabokov wrote in his novel The Defense, "By an implacable repetition of moves it was leading once more to that same passion which would destroy the dream of life."
The "angry opposition" sense of the word is perhaps more common today.