An older form meaning disagreement, conflict, or a lack of harmony between sounds, ideas, or people.
From Latin dissonantia, with the Anglo-Norman legal suffix -cy (used to form abstract nouns from adjectives). This variant declined in use as 'dissonance' became the standard form.
This word is almost extinct in modern English, but you can still find it in legal documents and philosophical texts from the 1600s—it's a linguistic fossil showing how English words compete and one version usually wins out.
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