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an unknown quantity
A person or thing that is unpredictable and thus difficult to anticipate. We're still revising our new product, so its potential success is an unknown quantity. This is her first year teaching, so her ability to connect with her students is still an unknown quantity.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
Fig. a person or thing about which no one is certain. (*Typically: be ~; become ~.) John is an unknown quantity. We don't know how he's going to act. The new clerk is an unknown quantity. Things may not turn out all right.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
unknown quantitya person or thing whose nature, value, or significance cannot be determined or is not yet known.
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