Cuss is the Job skill for Wit. You can get job skill after you rebirth and whichever job you choose for RB, is a different ability you will get. Cuss amplifies damage you can do to a target (Like shrink, but better), so many people will use it to train a new account and sometimes sell it to other players for Item Mall. You shouldn't worry about cuss burst unless you plan to buy it or until you are rebirthed.