The quality or state of being accessary; the characteristic of being supplementary or of serving as an accomplice.
From accessary plus -ness (making it into an abstract quality noun). This follows the standard English pattern for creating nouns of quality.
The suffix '-ness' is English's superpower for turning adjectives into abstract nouns—'accessariness' means the essential quality of being auxiliary or supplementary, and it's rarely used because we just say 'auxiliary' instead!
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