That was a gag from the Twelfth Doctor's first episode. The Twelfth Doctor is played by the actor Peter Capaldi, and he's got rather intense eyebrows. At one point in the episode, while he's still a little loopy from having just regenerated, he looks at his reflection and declares his eyebrows as attack eyebrows wishing to become independent and start their own nation, while a nearby homeless man cowers in fear before the ranting scotsman in front of him.