Buster was a nickname.
In 1986, after James T. Kirk assured her he and Spock were trying to help George and Gracie, Gillian Taylor retorted, "The hell you were, buster." (Star Trek IV: The Voyage Home)
In 2268, Leonard McCoy shouted, "You tell him, buster" to the gladiator Achilles after the 892-IV native encouraged Spock to fight. (TOS: "Bread and Circuses")
Later that year, after Captain Kirk told Jojo Krako that the crew of USS Enterprise numbered over four hundred, Krako taunted, "That's your story, buster." (TOS: "A Piece of the Action")