unsolved
adj. B2 Upper Intermediate US //ənˈsɑɫvd// UK //ʌnsˈɒlvd// un·solved
adj. describing a mystery, crime, or problem that has not been explained or fixed yet.
adj. referring to a problem, mystery, or crime for which no solution or explanation has been found. Often used to describe cold cases or mathematical paradoxes.
The mystery of the missing keys remains unsolved.
Police are still looking for new evidence in the unsolved robbery that took place last winter.
Despite decades of rigorous investigation and the application of modern forensic technology, the identity of the culprit in the 1974 case remains one of the city's most famous unsolved mysteries.
From un- + solved.
Usage
Typically used attributively before a noun or predicatively after a linking verb like 'remain' or 'stay'.