E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
means one called to assist clients in a court of law. (Latin, advocre.)
The Devils Advocate. One who brings forward malicious accusations. When any name is proposed for canonisation in the Roman Catholic Church, two advocates are appointed, one to oppose the motion and one to defend it. The former, called Advocatus Diaboli (the Devils Advocate), advances all he can against the person in question; the latter, called Advocatus Dei (Gods Advocate), says all he can in support of the proposal.