Say what you will about Richard Sherman, the outspoken cornerback for the Seattle Seahawks, he’s right that the word “thug” has become an “accepted way of calling somebody the N-word.” You saw it, for instance, during the Trayvon Martin controversy, where defenders of the shooter, George Zimmerman, were quick to label Martin a “dangerous thug” for ordinary teenage behavior, like cursing or smoking marijuana.

The same is true of the right-wing fever swamps, where “thug” is the favored insult for Barack Obama. It comes quickly from the mouths of Rush Limbaugh—who is quick to call the president a “Chicago thug”—Michele Bachmann—who attacks anodyne political behavior as “thuggery”—and Karl Rove, who told Fox News that the president looks like “some kind of political thug” because of comments he made about the Supreme Court.

Beyond this, the conservative blogosphere is filled with people who routinely describe Obama as a “Chicago thug” or attack him for bringing “Chicago politics” to the White House.

Obviously, you can’t attribute all of this to racial animus. But there’s no question that the word “thug”—and it’s modifier, “Chicago”—has a distinct racial connotation, and is meant to convey a particular image—of Obama as a menacing young black man. And indeed, it would be naive to say that the Limbaughs of the world aren’t aware of this, especially when they’re happy to accuse Obama of all sorts of anti-white racism.

Last week, as reported by David Remnick of The New Yorker, Obama attributed some of his political difficulties to a subset of Americans who oppose the idea of a black president. This sparked a predictable torrent of right-wing outrage, but he wasn’t wrong. There are Americans who oppose Obama out of racial animus. And when provocateurs like Glenn Beck or Limbaugh accuse the president of having a “deep-seated hatred of white people,” that’s who they’re aiming for.

In other words, these constant declarations of Obama’s “thuggery” are a dog-whistle—an attempt to make the president a dangerous “other” without resorting to the kind of language that earns wide public condemnation. It allows the speaker to indulge racial taboos without ever crossing the line.

It’s a nifty trick, and one that we should expect to see for the remainder of Obama’s presidency, and whenever a famous African American—like Sherman—crosses some invisible line of perceived misconduct.