The Answer:

The University of North Carolina offers a couple of explanations. In one, the nickname goes back to the Revolutionary War; what's now known as the Tar River had tar poured into it to slow down British troops. Those who forded the river found their feet covered in tar when they emerged, leading to the "tar heel" nickname.

Another claims that the nickname was given during the Civil War, in which Robert E. Lee was reported as saying, with regard to steadfast North Carolinan soldiers, "There they stand as if they have tar on their heels."

The UNC site also has a more colorful version of that story, which you can read using the above link.

—The Editors