Typically used on internet chat forums, to Godwin a thread or topic is to somehow make reference to Nazi Germany, or Nazis in general, which has the effect of derailing the thread and completely changing the intended subject.
SexyDude: I disagree, BMWs are better cars than Acuras.
xTroll69: BMWs have their roots in Nazi Germany.
Carman34: That's not true, the Nazis preferred to...etc etc
SexyDude: You just totally godwin'd this thread! Good work!
person 1: I think <insert name here> should win the election.
person 2: He's a filthy Nazi!
person 1: No he's not!
person 2: Shut up you f***ing Nazi!