What does unreliable mean?
Unreliable describes something that cannot be trusted to work consistently, provide accurate results, or fulfil its expected function. The word is especially common when discussing sources, information, witnesses, vehicles, equipment, signals, and services. Saying that a source is unreliable suggests that its claims may be inaccurate or deceptive; saying that a system is unreliable usually means that it fails, varies unpredictably, or does not perform consistently. The term carries a generally negative practical connotation because reliability is essential for making plans and judging other people’s claims. “Unreliable” is often stronger and more direct than “inconsistent,” which can describe regular variation as well as faultiness. It is distinct from a statement that something is doubtful or merely unknown: unreliability implies repeated failure or an established lack of dependability. Used carefully, it identifies an important quality when assessing evidence, technology, transport, or personal testimony.
adjectiveNot able to be trusted or relied upon to work, perform, or provide accurate information. An unreliable person, source, system, or service cannot be depended on.
- Not able to be trusted or depended upon; lacking consistency, dependability, or accuracy.
"The bus service is notoriously unreliable during storms."
"The research depended on an unreliable witness, so its conclusions had to be reconsidered."
"Because the signal is unreliable, the device automatically switches to its backup connection."
When a system fails often enough, “unreliable” becomes more than a criticism—it becomes a design brief.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does unreliable come from?
Unreliable was formed in English by adding the negative prefix un- to reliable, and it was in use by the 17th century. Reliable itself comes from rely plus the suffix -able, a pattern meaning “able or worthy of being relied upon.” The related words rely and reliance continue to express the idea of dependence on someone or something.
Related word forms
How unreliable is actually used
Commonly used for people, sources, vehicles, systems, signals, and services. It can describe both serious failures and everyday inconvenience, from an unreliable prediction to a transportation network that is unreliable during rush hour.