unreliable

unreliable — adjective

1. If a person or thing is unreliable, you cannot trust or depend on them to do wha

1.adjectiveB1
Definition

If a person or thing is unreliable, you cannot trust or depend on them to do what is expected — for example, a friend who never keeps promises, a car that breaks down often, or a news website that publishes false stories.

Example

Sofia's old car was so unreliable that she had to take the bus to work every day.

so + adjective + that-clause showing result

The news report came from an unreliable website full of made-up facts.

unreliable + noun (source, website, data)

Common collocations
  • unreliable source
  • unreliable data
  • unreliable witness
  • unreliable car
  • unreliable internet
  • prove unreliable
  • highly unreliable
  • notoriously unreliable
Synonyms
  • undependable

    stronger synonym for people and machines; more common in American English

  • untrustworthy

    focuses on honesty rather than performance; used for people and information, not machines

  • unpredictable

    broader — includes wildly varying behaviour beyond just failure; neutral or positive in some contexts ('an unpredictable plot')

Antonyms

Usage Note

Commonly applied to people (friends, workers, sources), machines (cars, computers), and information (data, news, studies). Can describe a temporary state ('The internet is unreliable today') or a fixed character trait ('He is an unreliable person').

Common Mistakes

The old bus is very untrustworthy.
The old bus is very unreliable.
'untrustworthy' describes people or information (moral/truth dimension); 'unreliable' describes things that fail to function or perform as expected.

Word family

Synonym discussion

Sense: adjective/1

All three words describe something that cannot be relied on, but they differ in scope and emphasis. 'Undependable' is the closest synonym and is interchangeable in most contexts, though it is slightly more common in American English and often describes people or machines ('an undependable car'). 'Untrustworthy' carries a moral judgement — it suggests dishonesty or a lack of integrity, so it fits people and information but not mechanical objects ('an untrustworthy source' sounds natural; 'an untrustworthy laptop' does not). 'Unpredictable' is broader: it includes failure but also covers wild variation that could be exciting or creative ('an unpredictable film'). To say a worker is 'unreliable' means they do not show up or finish tasks; to say they are 'unpredictable' could mean their work quality swings wildly. For most everyday situations — broken machines, late friends, shaky data — 'unreliable' is the safest and most natural choice.

undependableuntrustworthyunpredictable ↗

Common collocations

Nouns

Adverbs

Phrases

  • prove unreliable
  • deem unreliable
  • dismiss as unreliable

Etymology

LatinFrenchEnglish

Formed in English by adding the negative prefix 'un-' to 'reliable', which comes from the verb 'rely' (Middle English 'relien', from Old French 'relier' — to fasten or bind together, from Latin 'religare': 're-' meaning back + 'ligare' meaning to bind). The core image is of something 'bound back' to you — something you can count on because it is tied to your expectations. Adding 'un-' reverses that: something that is not tied down, not dependable.