BEWARE, YE foul-mouthed citizens of Fairfax County: The sensibilities of your police force are so delicate, their taste in language so virtuous, their ears so unsullied by rude speech that they regard the utterance of profanity as justification for a half-dozen of them to throw you to the ground, crush your head to the pavement with their knees, wrestle you into submission and arrest you.

A Fairfax ordinance seems to give the police carte blanche in this regard, proclaiming that "if any person profanely curse or swear or be drunk in public, he shall be deemed guilty of a Class 4 misdemeanor."

The trouble is, the ordinance, like the police policy, is flatly unconstitutional, as the Supreme Court and lower tribunals have ruled repeatedly. And the Fairfax County police chief, Edwin C. Roessler Jr., only encouraged police to overreact and use gratuitous violence by offering an unqualified defense of some officers' recent actions, even as the police opened an internal investigation of the incident that prompted the chief's remarks.

The incident in question involved Mike Stark, a journalist who, while covering a political rally in Annandale last weekend, was confronted by an officer who told him to get on the sidewalk. (He was barely off it.) In a video of the incident, Mr. Stark, who works for Shareblue, a left-leaning website, is seen protesting: "I'm a f---ing reporter doing my job."

Another officer informed him that if he swears again, "you're going to jail." Mr. Stark replied, "F--- this," whereupon the police officer pounced, threw him to the ground, and, joined by reinforcements pinning him to the ground, handcuffed and arrested him. Ultimately, Mr. Stark was charged with disorderly conduct and avoiding arrest — not with swearing.

Mr. Stark may not have been wise in his dealing with the police. It's equally true that the officers' response was unprofessional, at the least. A citizen's lack of social polish or politesse does not justify the officers' use of violence. There's no excuse for verbally abusing police officers, but in real life it happens plenty, and if police officers can't take foul language directed at or used around them now and then, they're in the wrong line of work. (The same goes for journalists.) At no time did Mr. Stark threaten the officers or anyone else.

Courts have been clear that, as Justice John Marshall Harlan II wrote for the Supreme Court in Cohen v. California, a free-speech case in 1971, "one man's vulgarity is another man's lyric." In that case, the court ruled that the First Amendment protected an individual's right to wear a jacket inscribed with the words "F--- the Draft." And while the court has exempted "fighting words" from First Amendment protections, those are generally considered language meant to incite others to violence. Mr. Stark may have been irascible and ill-tempered, but he was hardly trying to start a riot.

Fairfax County police have opened an internal investigation of Mr. Stark's arrest. That's a good thing. Mouthing off is not cause for arrest, let alone physical abuse by police.