There is a “fighting words” exception to the First Amendment, which has been narrowed by courts so far as honor culture has declined that it’s mostly non-operative today. That isn’t a defense for someone who attacks someone who says something, but it did recognize that there was a social culture at one point of not being able to just outright insult someone without them having a social obligation to respond violently, so it permitted laws against such insults.

https://en.wikipedia.org/wiki/Fighting_words

Fighting words are spoken words intended to provoke a retaliatory act of violence against the speaker. In United States constitutional law, the term describes words that inflict injury or would tend to incite an immediate breach of the peace.

The fighting words doctrine, in United States constitutional law, is a limitation to freedom of speech as protected by the First Amendment to the United States Constitution.

In 1942, the U.S. Supreme Court established the doctrine by a 9–0 decision in Chaplinsky v. New Hampshire.[1] It held that “insulting or ‘fighting words’, those that by their very utterance inflict injury or tend to incite an immediate breach of the peace” are among the “well-defined and narrowly limited classes of speech the prevention and punishment of [which] … have never been thought to raise any constitutional problem.”

Chaplinsky decision

Walter Chaplinsky, a Jehovah’s Witness, had purportedly told a New Hampshire town marshal who was attempting to prevent him from preaching that he was “a God damned racketeer” and “a damned fascist” and was arrested. The court upheld the arrest and wrote in its decision that:

There are certain well-defined and narrowly limited classes of speech, the prevention and punishment of which have never been thought to raise any Constitutional problem. These include the lewd and obscene, the profane, the libelous, and the insulting or “fighting” words – those which by their very utterance inflict injury or tend to incite an immediate breach of the peace. It has been well observed that such utterances are no essential part of any exposition of ideas, and are of such slight social value as a step to truth that any benefit that may be derived from them is clearly outweighed by the social interest in order and morality.

— Chaplinsky v. New Hampshire, 1942[1]

The idea here is that you can’t just go around calling someone a fascist without expecting them to immediately shoot you or otherwise hurt you in response, so to avoid that, it’s constitutionally-acceptable for legislators to pass laws to prohibit someone from calling someone else a fascist like that.

Over the years, it’s been steadily narrowed by a series of court decisions.

Post-Chaplinsky

The Court has continued to uphold the doctrine but also steadily narrowed the grounds on which fighting words are held to apply. In Street v. New York (1969), the Court overturned a statute prohibiting flag-burning and verbally abusing the flag, holding that mere offensiveness does not qualify as “fighting words”. In similar manner, in Cohen v. California (1971), Paul Robert Cohen’s wearing a jacket that said “fuck the draft” did not constitute uttering fighting words since there had been no “personally abusive epithets”.

In Brandenburg v. Ohio (1969), even speech such as “Bury the niggers” and “Send the Jews back to Israel,” was held to be protected speech under the First Amendment in a per curiam decision. In addition, despite the speech being broadcast on network television it did not direct to incite or produce imminent lawless action nor was it likely to produce such action.

In 1972, the Court held that offensive and insulting language, even when directed at specific individuals, is not fighting words:

• Gooding v. Wilson (1972): “White son of a bitch, I’ll kill you.”[2]
• Rosenfeld v. New Jersey (1972): “mother fucking.”[3]
• Lewis v. City of New Orleans (1972): “god damn mother fucking police.”[4]
• Brown v. Oklahoma (1972): “mother fucking fascist”, “black mother fucking pig”. Found constitutional because the “speech [may] have been anticipated by the audience.”[5]

In Collin v. Smith (1978) Nazis displaying swastikas and wearing military-style uniforms marching through a community with a large Jewish population, including survivors of German concentration camps, were not using fighting words.

Texas v. Johnson (1989) redefined the scope of fighting words to “a direct personal insult or an invitation to exchange fisticuffs” in juxtapose to flag burning as symbolic speech.[6]

In R.A.V. v. City of St. Paul (1992) and Virginia v. Black (2003), the Court held that cross burning is not ‘fighting words’ without intent to intimidate.

In Snyder v. Phelps (2011), respondents’ counsel argued that the Court’s definition of fighting words required immediacy, imminence, intent and proximity. Justice Ginsburg stated that the Court had rejected spreading the concept beyond words that immediately trigger an instinctive reaction.[7] The Court held that even “outrageous” and “hurtful speech” such as: “God Hates the USA/Thank God for 9/11”, “America is Doomed”, “Don’t Pray for the USA”, “Thank God for IEDs”, “Thank God for Dead Soldiers”, “Pope in Hell”, “Priests Rape Boys”, “God Hates Fags”, “Fags Doom Nations”, “You’re Going to Hell,” and “God Hates You” is to be considered public debate, particularly when conducted on public land, and must enjoy “special” First Amendment protection. Lone dissenting Justice Samuel Alito likened the protests of the Westboro Baptist Church members to fighting words and of a personal character, and thus not protected speech. The majority disagreed and stated that the protesters’ speech was not personal but public, and that local laws which can shield funeral attendees from protesters are adequate for protecting those in times of emotional distress.