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Adjective: undependable  ,ún-di'pen-du-bul
  1. Not worthy of reliance or trust
    "don't rely on my undependable friend to do it for you";
    - unreliable, flaky [informal], flakey [informal]
     
  2. Liable to be erroneous or misleading
    "an undependable generalization";
    - unreliable

See also: erratic, fallible, temperamental, uncertain, unsound, untrustworthy, untrusty

Antonym: dependable