Synonyms of 'unknown quantity'
▷ See quantity
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If you describe something or someone as an enigma, you mean they are mysterious or difficult to understand.
This country remains an enigma for the outside world.
A mystery is something that is not understood or known about.
The source of the gunshots still remains a mystery.
Thesaurus for unknown quantity from the Collins English Thesaurus
If you say that someone or something is an unknown quantity , you mean that not much is known about what they are like or how they will behave .