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*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.
See also: quantity, unknown

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.
See also: quantity, unknown

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.
See also: quantity, unknown
References in classic literature ?
The invisible Prince recognised these dead men as Kings of two large islands near his own home, but the names of the Princesses were unknown to him.
asked the marquis, signing toward the unknown woman.
She prevailed upon him to leave the country; but to leave the country, to fly together, to reach another part of France, where they might live at ease because unknown, money was necessary.
Then there is the stronger reason for my remaining," said the unknown.
Upon her he had lavished, unknown to himself, all the reverence and respect and love that a normal English boy feels for his own mother.
Not expecting to come on the enemy down by the stream, and having stumbled on him in the fog, hearing no encouraging word from their commanders, and with a consciousness of being too late spreading through the ranks, and above all being unable to see anything in front or around them in the thick fog, the Russians exchanged shots with the enemy lazily and advanced and again halted, receiving no timely orders from the officers or adjutants who wandered about in the fog in those unknown surroundings unable to find their own regiments.
she demanded, after a long silence; each word as it came into being seemed to shove itself out into an unknown sea.
Murray, of Horton Lodge, near O , about seventy miles from our village: a formidable distance to me, as I had never been above twenty miles from home in all the course of my twenty years' sojourn on earth; and as, moreover, every individual in that family and in the neighbourhood was utterly unknown to myself and all my acquaintances.
I can assure you that before many months are past you will know more people than ever you have spoken to in your life before - men whose names have been household words to you and nothing else will be calling you 'old chap' and wanting to sell you horses, and women, who last week would look at you through lorgnettes as though you were a denizen of some unknown world, will be lavishing upon you their choicest smiles and whispering in your ear their 'not at home' afternoon.
No more doughty nor daring was Columbus, venturing all that he was to the unknown, than was Jerry in venturing this jungle-darkness of black Malaita.
The unknown bore him on he knew not to what frightful hurt, and he yelped and ki-yi'd unceasingly.
Here is a letter--a long delightful letter--from the Engadine, left at the door by some gentleman unknown.
AN Anarchist Orator who had been struck in the face with a Dead Cat by some Respector of Law to him unknown, had the Dead Cat arrested and taken before a Magistrate.
This slender boy was the Unknown, vested with all the menace of the Unknown.
Sir," said the unknown, "you pretend that our satellite is inhabited.