It's interesting why do people always easily memorise foreign swear words, but when it comes up to a normal vocab, they have difficulties  Maybe it's because they want to use them right away as soon as they step on a foreign land  Or they want to understand these words if someone will ever use one in front of them... Or maybe because the forbidden fruit is suit, we are always attracted to something that is kind of prohibited. 
One of our future students who's applyed for a scholarship wrote in her essay: "It all started when I was little and went to summer camp with a youth organization in Belgium. There, I met a Russian girl who was staying in a Belgian host family. The first Russian word she tought me was a swear word! She told me that if someone ever treated me badly, I had to call them ‘durak’."
So, of course we hope that it will not be your first Russian word, but the one you definitely need to know ДУРАК (for a guy) or ДУРА (for a girl) is actually not that bad. Let's say, it's the sweetest way to abuse somebody As a matter of fact, in many Russian folk fairy tales the main character is a ДУРАК. Usually it's ИВАН-ДУРАК (Ivan the stupid).
Hope you'll never have to use Russian swear words, but if you do, you might wanna say something stronger than just ДУРАК