The Collaborative International Dictionary of English v.0.48:

Giaour \Giaour\, n. [Turk. giaur an infidel, Per. gawr, another
   form of ghebr fire worshiper. Cf. Kaffir, Gheber .]
   An infidel; -- a term applied by Turks to disbelievers in the
   Mohammedan religion, especially Christrians. --Byron.
   [1913 Webster]