The quality or state of being expiatory; the character of serving as atonement.
From expiatory + -ness suffix, creating an abstract noun describing the quality of expiatoriness. This is a highly specialized term formed through standard English derivation rules.
Creating '-ness' nouns from adjectives is how English lets us talk about abstract qualities—'expiatoriness' is rare, but it's the word you'd use to discuss the philosophical concept of what makes something truly 'expiatory' in nature.
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