What is the origin of the phrase Devil's advocate?

"advocatus diaboli" (latin term used in canon law) - in beatification or canonization process it's a person (priest?), who has to discover every information which could tell anything opposite to any evidence of sanctity.

The goal of a. d. is to make postulators (the guys who are pro-sanctity) work harder.

So, in the common language, the term "devil's advocate" is used to describe someone, who is defending something inappropriate ro unsuccesful (sometimes to make others defend the good or bright ideas harder).