In a manner that can be accused; in a way that is subject to accusation or blame.
From accusable plus the adverb suffix -ly. This rare adverb form extends the adjective's meaning into the adverbial realm for modifying verbs and clauses.
This is an incredibly rare word because it's hard to use—you'd almost never say 'he accusably did something,' but in super formal legal language it might appear!
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