The "fighting words" doctrine says that there are certain kinds of words that are not protected by the First Amendment. This is because the use of these words is not really speech -- it is more like a verbal assault. This doctrine was first set out in Chaplinsky v. New Hampshire in 1942. In this case, the Court held that calling a policeman a "fascist" was not real speech. It held that that word and words like it were not protected because they
by their very utterance inflict injury or tend to incite an immediate breach of the peace.
This doctrine is still technically in effect. However, cases since then have watered it down to the point where it is hard to know what a fighting word would actually be. For example, the Court has ruled against "hate speech" laws that prevent people from displaying symbols or using words meant to make people hate those of another race (this is the RAV case in the link below).
So it is not clear what words would actually be deemed fighting words and therefore not worthy of protection.