unknown quantity noun [C usually singular]
a person or a thing whose abilities, powers or effects are not yet known:
The third candidate for the party leadership is a relatively unknown quantity.

[TahlilGaran] Advanced Learner's Dictionary ▲

unknown quantity
a person or thing which nobody knows much about
The new mayor was an unknown quantity and nobody knew what to expect.

[TahlilGaran] English Idioms Dictionary ▲

unknown quantity
n. Someone or something whose value and importance are not known, especially in a certain situation, time or place; a new and untested person or thing.
What we would find if we could fly to the moon is an unknown quantity.
The new player is still an unknown quantity. We'll find out how good he is in the game.

[TahlilGaran] English Idioms Dictionary ▲