The Dunlendings are a primitive peoples living in Middle-earth as depicted in J.R.R. Tolkein's novel, The Lord of the Rings. They live near the people of Rohan but fear and hate them.
The exact origins of the Wild Men are somewhat mysterious, but is is apparent that they were already living in Middle-Earth when the Númenóreans first arrived in those lands. They detested the men of Rohan who eventually settled near them, leading to on and off violence that steadily forced the Wild Men tribes deeper and deeper into the harsh lands of Dunland. There they grew hardy in the windy and rough environment, but were also ever embittered towards Rohan's people.
Incited into violence by Saruman the White, the Wild Men join him and assist his Orcs and Uruk-hai in attacking the lands of Rohan and slaughtering their people. Eventually, Rohan's people become desperate enough to retreat to the refuge of Helm's Deep.
In the books, they assist the Orcs and Uruk-hai in attacking Helm's Deep, but they and the Uruk-hai lose to Rohan. Conversely, in the continuity of the movies, the Wild Men are not present at the Battle of Helm's Deep, having been earlier pounded by either mounted companies of men who patrol the lands of Rohan to keep them safe, or perhaps even the Rohirrim that had been mustered by Eomer and Gandalf the White.
Either way, it would be the last time the Wild Men made a concentrated attack on Rohan's people. After the battle is lost, the Uruk-hai are devoured by the trees while the Dundenlings are rounded up, and spared by the men of Rohan under the condition that they not rise up against them again. Touched by this display of mercy and honor, the Wild Men honor the agreement, and peace prevails.