An interesting and likely correct application of current “fighting words” law, from In re E.O., 2013 WL 1339930 (Kan. Ct. App. Mar. 29, 2013) (nonprecedential):
Although the phrase “fuck you” is offensive and profane, when it is said once by a juvenile to an adult, police officer or not, who is 150 feet away, while the juvenile is walking away and the phrase is unaccompanied by any threatening motions, the phrase does not rise to the level of words which “by their very utterance inflict injury or tend to incite the listener to an immediate breach of the peace.” Thus, even considering the evidence presented in the light most favorable to the prosecution, the evidence failed to show that E.O.’s speech constituted fighting words.
The Court has held that face-to-face personal insults that have a tendency to cause an imminent fight can be punished as fighting words. Lower courts have concluded that such insults aren’t fighting words when sent in an e-mail or a letter, because no imminent fight is likely. This case involves something in between, and is thus an interesting data point (though not a binding precedent).