Relating to or characteristic of people secretly planning something harmful or illegal; involving a conspiracy.
From conspirator (one who conspires) + -ial (adjective suffix). This is the most common modern English adjective form of conspiracy.
A 'conspiratorial whisper' or 'conspiratorial tone' specifically means someone is speaking like they're sharing a secret plot—it's become a word we use to describe *the manner* of communication, not just the content.
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