Used with humor, as a simple statement, in a conversation when you strongly disagree with the majority. Originally a French term used in psychiatry to describe a mental disorder in which two or more associated persons share the same delusion. In other words, you are saying "The two of you share the same delusion." Translated "madness between two."
Two others state "G. Dub is an excellent public speaker." In your disagreement, you respond with "Folie a deux!"