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unknown
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.
*unknown quantity
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.
an unknown quantity
if someone or something is an unknown quantity, you do not know much about them or what effect they will have in the future Turner may do well in the election, though he is an unknown quantity as a campaigner. The new computer system is still an unknown quantity for our department.
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.