Commonly thought to be a word of its own, Cussin or Cussing is saying the word "curse" with an accent.
Most common in the southern US, where the southern accent originated from Scottish/Irish dialect. Although it is most common in the south, since most people think its a word of its own, it has spread throughout the entire US.
If you say "cursing" with an Irish, Scottish, or even a British accent, it sounds similar to "cussing". Now give it a nice southern drawn and drop the G off for good measure. Cussin.