unknown quantity
noun [ C usually singular ] uk/ˌʌn.nəʊn ˈkwɒn.tɪ.ti/ us/ˌʌn.noʊn ˈkwɑːn.t̬ə.t̬i/
C2 未知数（尚未被人了解的人或事物）
a person or a thing whose abilities, powers, or effects are not yet known
The third candidate for the seat is a relatively unknown quantity. 竞选党领导的第三个候选人的具体情况相对而言还不甚清楚。