Here are two interesting explanations:
Basurman, or Bursurman, mutilated from Mussulman, is an archaic and contemptuous designation for Mohammedans and in general for all who do not profess the Greek Orthodox faith.
Dubnow, Simon and Friedlaender, Israel, History of the Jews in Russia and Poland 35 n.2 (1916) Link
The term Bursurman for Muslims in the old Russian annals was from the language of Khwarazm in the Arabo-Persian script current until the Mongol conquest of the Volga region in the 13th century.
Rahul, Ram, March of Central Asia 156 (2002) Link