|Work can be very rough when your co-worker is very smooth. If you continue to let your job be his dumping ground, you will ultimately be buried in his castoffs. The problem is not only your smooth co-worker, but also your manager who is seemingly smitten with him.|
Your first step is to be more assertive when your co-worker starts to jettison his junk toward you. Be clear and direct in telling him that the work in question is his, and be certain he understands that you are not going to do it.
If he then runs to your manager, you should insist on a meeting with both of them. It's time to clear the air as well as clear your desk of his responsibilities. In this meeting, be prepared to talk business and not personalities. The objective is to come to a more formalized agreement and understanding regarding job responsibilities.
If your manager cannot get beyond your co-worker's smoothness, then you should get beyond this manager and see what his or her manager has to say about this.