unreliable

Someone unreliable can't be trusted to do something. Things can be unreliable too, like a bike with a wobbly wheel.

The word "rely" is a clue to what unreliable means. When you can rely on something, you can count in it — it's reliable. On the other hand, you'd better not count on an unreliable person. People can be unreliable because they're dishonest, always late, bad at their job, or just inconsistent. Things can be unreliable because they're broken or old. A train schedule can be unreliable if the trains are never on time. Unfortunately, there's a lot of stuff in life that's unreliable, so be careful.

Definitions of unreliable
  1. adjective
    not worthy of reliance or trust
    “in the early 1950s computers were large and expensive and unreliable
    synonyms: undependable
    erratic, temperamental, wayward
    likely to perform unpredictably
    uncertain
    not consistent or dependable
    unsound
    of e.g. advice
    untrustworthy, untrusty
    not worthy of trust or belief
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    Antonyms:
    dependable, reliable
    worthy of reliance or trust
    certain, sure
    reliable in operation or effect
    tested, time-tested, tried, tried and true
    tested and proved to be reliable
    undeviating
    used of values and principles; not subject to change; steady
    trustworthy, trusty
    worthy of trust or belief
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  2. adjective
    liable to be erroneous or misleading
    synonyms: undependable
    fallible
    likely to fail or make errors
  3. adjective
    dangerously unstable and unpredictable
    “an unreliable trestle”
    synonyms: treacherous
    dangerous, unsafe
    involving or causing danger or risk; liable to hurt or harm
  4. adjective
    lacking a sense of responsibility
    Synonyms:
    irresponsible
    showing lack of care for consequences
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