“This sounds like a road map for threatening a spouse and getting away with it,” Justice Alito said. “You put it in rhyme and you put some stuff about the Internet on it and you say, ‘I’m an aspiring rap artist.’ And so then you are free from prosecution.”

The Supreme Court has said that “true threats” are not protected by the First Amendment, but it has not been especially clear about what counts as such a threat. Justice Anthony M. Kennedy said the term itself was unhelpful.

“I’m not sure that the court did either the law or the English language much of a good service when it said ‘true threat,’ ” he said. “It could mean so many things.”

The question for the justices in Monday’s case, Elonis v. United States, No. 13-983, was whether prosecutors had done enough to prove Mr. Elonis’s intent. Mr. Dreeben, the government lawyer, said the words and their context were enough.

The standard proposed by the government, he said, would hold people accountable “for the ordinary and natural meaning of the words that they say in context.”

Mr. Elwood said more was required. Ideally, he said, prosecutors should have to prove that the speaker’s purpose was to threaten someone. Failing that, he said, prosecutors should at least have to prove that the speaker, whatever his or her purpose, knew “that it’s a virtual certainty” that someone would feel threatened.

The lower courts sided with Mr. Dreeben, the government lawyer. All the prosecution had to prove, the trial judge ruled, was that a “reasonable person” would foresee that others would view statements “as a serious expression of an intention to inflict bodily injury or take the life of an individual.”