The be-all end-all of insulting names to call someone. More vulgar than any other word you could possibly think of. It means the cum of Kane, one of the top 5 worst people on Earth. One would be hard pressed to find a situation in which this would be legitimate to use.
Has nothing to do with South Park, even though South Park is used for the example.
Hypothetically, after the South Park episode...
Scott: "Cartman, you kanecum, you made me eat my parents and the pubes of everyone in town in front of my favorite band!"
Stan: "Dude, think about what you're saying! It wasn't THAT bad!"