An ermine violin was a hypothetical musical instrument mentioned by Spock in a 2268 conversation with Leonard McCoy.
When countering Spock's criticism that tribbles seemed to have no practical use, McCoy seemed to feel it was enough that the creatures were "nice, soft, furry and they make a pleasant sound." Spock objected, saying the doctor's statements would also apply to an ermine violin, but he saw no value in possessing either. (TOS: "The Trouble with Tribbles")