The finger, also known as flipping the bird, is an offensive hand gesture from Earth Human culture, particularly the punk subculture. It involved holding up one's middle finger and showing 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 Kirk asked the punk to turn off a boombox which the punk had. (Star Trek IV: The Voyage Home)