Quote Originally Posted by randir
Бусурман_ - иноземец ...
The plural is бусурмане, and the singular is бусурман, not бусурманин.

Quote Originally Posted by kid
What would be a cutting remark, in Russian, in response to someone who has used that word to me?
This word is not so much offensive, and depending on context, it could be even a joke.
But if you want, you can tell him, for example: Поскреби себя, и найдешь татарина. Although this answer maybe too clever.