COBUILD Advanced British English Dictionary1. graded adjective & adjective [usu ADJ n]If someone does something such as speak or smile in a conspiratorial way, they do it in a way that suggests they are sharing a secret with someone. ⇒ His voice had sunk to a conspiratorial whisper. ⇒ When I went to collect the car, Bill met me with a conspiratorial grin.conspiratorially
graded adverb [ADV after v] ⇒ The officer leaned forward conspiratorially and said: 'I shouldn't worry about it, mate.'2. adjectiveSomething that is conspiratorial is secret and illegal, often with a political purpose. ⇒ There is nothing sinister or conspiratorial about the export licensing system. ⇒ ...a secret and supposedly conspiratorial party meeting.