Sujo = Dirty

                               Nasty = Nojento

                            For example lets imagine that Dale has just prepared his first feijoada and the result resembles the aftermath of the chernobyl nuclear plant burndown. Then an unadvised guest at his house tastes the feijoada and simply says "nasty"!

                            By using nasty in this context he is simply saying "que nojo" or "nojento".

                            If you want to use this word (nasty) and you don´t have access to dale´s feijoada just eat any kibe in a bus station on a late night stop . If you want the nastiest kibe known to mankind take the belo horizonte-brasilia bus route which has three stops before reaching brasilia. On the third stop you will find the worse kibe ever devised by humans. NASTY!!! 

                           There is also a sexual connotation for nasty as was explained in dale´s reply previous to this one.