English

Noun

unknown quantity ‎(plural unknown quantities)

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

Synonyms

• blank canvas, enigma