The Collaborative International Dictionary of English v.0.48:

Abba \Ab"ba\ ([a^]b"b[.a]), n. [Syriac abb[=a] father. See
   Abbot.]
   Father; religious superior; -- in the Syriac, Coptic, and
   Ethiopic churches, a title given to the bishops, and by the
   bishops to the patriarch.
   [1913 Webster]