The quality of being adamant; supreme hardness, unyieldingness, or inflexibility of character or substance.
From 'adamant' + '-cy' (noun suffix creating abstract quality). A synonym for adamance that developed as an alternative nominal form.
English gives us both 'adamance' and 'adamancy' for basically the same meaning—these competing words show how languages evolve multiple ways to express the same idea until one wins out and the other becomes archaic.
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