The Collaborative International Dictionary of English v.0.48:

Unfrock \Un*frock"\, v. t. [1st pref. un- + frock.]
   To deprive or divest or a frock; specifically, to deprive of
   priestly character or privilege; as, to unfrock a priest.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unfrock
    v 1: divest of the frock; of church officials [syn: defrock,
         unfrock]