Conspiratorial (adjective)

1. Of or relating to a secret plan or agreement to do something illegal or harmful.
2. Of or relating to a secret plan or scheme to achieve a particular goal.
3. Relating to or suggestive of conspiracy.

Origin:

Late 19th century: from conspirator + -ial.

Examples:

1. He had a conspiratorial look in his eye, which made me suspicious.
2. The police discovered a conspiratorial note that provided evidence of the plot.
3. The conspiratorial whispers among the group raised suspicions among the authorities.
4. She had a conspiratorial smile as she shared her secret plan with me.
5. The conspiratorial tone in his voice made me think that he was involved in some illegal scheme.