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unknown quantity

An unpredictable person or thing, as in We don't know how the new pitcher will do—he's an unknown quantity. This expression comes from algebra, where it signifies an unknown numerical value. Its figurative use dates from the mid-1800s.
The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Examples from the Web for unknown quantity
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Historical Examples
  • I have let X equal the unknown quantity, but I don't know Y.

  • His blame was plentiful; his praise so rare as to be almost an unknown quantity.

    The First Violin Jessie Fothergill
  • He prefers, as his pseudonym implies, to remain an unknown quantity.

    From Jungle to Java Arthur Keyser
  • But how, in this case, to find out what was the unknown quantity?

    Uncanny Tales Various
  • For the amount of advertising that such a publisher is likely to do is an unknown quantity.

  • Tom, sober, silent and new among them, was an unknown quantity.

    Tom Slade on a Transport Percy Keese Fitzhugh
  • How about the jury, that unknown quantity of capricious predilections?

    Shadow and Light

    Mifflin Wistar Gibbs
  • At that time, as now, paper money was an unknown quantity in California.

    Buckskin Mose Buckskin Mose

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