Unknown Quantity Meaning

(idiomatic) A person or thing whose nature or value is a mystery.

Example: 1895, Stephen Crane, The Red Badge of Courage, ch. 1:
  Whatever he had learned of himself was here of no avail. He was an unknown quantity.
1907, The Bookman, vol. 25, p. 166 (Google snippet view):
  . . . the Evening Post of this city to refer to him recently as "the young Belgian painter," and to the general public he is an unknown quantity.
1982 Sep. 22, Henry Tanner, "Gemayel Brothers Assessed in Syria," New York Times (retrieved 9 Jan 2016):
  Amin Gemayel, it is felt here, is an unknown quantity and it is impossible to predict how he will act.
2015 Dec. 6, "Congratulations on 10 years, David Cameron - but don't rest on your laurels," Telegraph (UK) (retrieved 9 Jan 2016):
  [H]e was still an unknown quantity – a young, untested figure.