From Longman Dictionary of Contemporary Englishbe an unknown quantitybe an unknown quantityNOT KNOWif someone or something is an unknown quantity, you do not know what their abilities are or how they are likely to behave → unknown
Examples from the Corpus
be an unknown quantity• Swales said he had a lot of flair, but admitted he was an unknown quantity.• These arrangements are an unknown quantity and the administration may not turn out to be up to scratch.• Whatever it was they were after, it was an unknown quantity, unknown, that is, except for a lethal ferocity.• Barnes was an unknown quantity, without any clear prejudice on the nuclear issue. 