The finger, also known as flipping the bird, is an offensive hand gesture from Earth Human culture, particularly known to the punk subculture. It involved holding up one's middle finger and displaying it to another person.
In 1986, when a time traveling Admiral Kirk and Captain Spock were on a bus crossing the Golden Gate Bridge, a punk on the bus gave Kirk the finger after he asked him to turn down the volume on a boombox that he had, blasting a punk rock song from its' speakers. Immediately thereafter, said punk was made unconscious by Spock's Vulcan nerve pinch. (Star Trek IV: The Voyage Home)