In an accessary manner; as a secondary or supporting element; in the capacity of an accessary to a crime.
From accessary plus -ly (the adverb suffix). This creates an adverb form of the adjective meaning 'in the manner of an accessary.'
This adverb lets you describe how someone participated in a crime—'he was only accessarily involved' means he helped but didn't actually commit the main crime, which is a crucial legal distinction!
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