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an unknown quantity
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.
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.