unreliable
adj. · n.-
1 not trustworthy or consistent (adj.) B1 Intermediatenot able to be trusted or depended on to do something well or to be true.
lacking reliability; prone to failure, inconsistency, or inaccuracy.
ExampleThe bus service is very unreliable, so I usually walk to work instead.
ExampleThe witness was deemed unreliable by the prosecution after several major inconsistencies were discovered in her initial statement.
-
2 a person or thing that fails (n.) C2 Proficiency Formala person or thing that you cannot depend on.
an entity, whether a person or a mechanical system, that consistently fails to perform as expected.
ExampleAmong all the new machines we bought, this one has been a total unreliable.
ExampleThe report identified the aging generator as a chronic unreliable, recommending its immediate replacement to avoid further power outages.
UsageThis noun use is much rarer than the adjective form and typically appears in formal or technical reports.
From un- + reliable.