There is usually no mistaking the act or intent of extending a middle finger.

Take John Swartz, for example. In May 2006, Mr. Swartz was a passenger in a car in a rural part of upstate New York when he spotted a police car that was using a radar speed-tracking device.

Mr. Swartz, a Vietnam veteran and retired airline pilot, acted on instinct to show his displeasure: he extended his right arm outside the passenger’s side window, and then further extended his middle finger over the car’s roof.

The reaction was swift. The officer followed the car; words were exchanged; backups were called; and Mr. Swartz was arrested on a charge of disorderly conduct.

He later filed a civil rights lawsuit, and although a lower court judge dismissed the case, the prestigious United States Court of Appeals for the Second Circuit in Manhattan reversed that decision on Thursday, ruling that Mr. Swartz’s lawsuit can go forward.