‘Devil’s Advocate’ is someone who pretends, in an argument or discussion, to be against an idea or plan that a lot of people support, in order to make people discuss and consider the idea in more detail.
Example of use: “So you believe that Canadians are friendly? I’m going to be the devil’s advocate and say that they’re unfriendly! ”
The term 'Devil's advocate' was brought into English in the 18th century from the mediaval Latin expression 'advocatus diaboli'.