not reliable; not to be relied or depended on.

1830–40; un-1 + reliable

unreliability, unreliableness, noun
unreliably, adverb

undependable, irresponsible, untrustworthy.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
unreliable (ˌʌnrɪˈlaɪəbəl)
not reliable; untrustworthy: an unreliable witness

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1840, from un- (1) "not" + reliable. First recorded in De Quincey, who said it was coined by Coleridge.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Everyone has heard that memories can be unreliable, but many of us don't
  appreciate the extent to which that is true.
Artificial sprinklers were set up to mimic the natural spray of the falls, but
  they were unreliable.
He had tinkered with it long enough to think it might not be totally unreliable.
Moreover, prolonged nursing has been found to be unreliable as a contraceptive.
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