Not able to be trusted or depended upon; inconsistent or undependable in character or performance.
From un- (negative prefix) plus reliable (from rely, from Old French relier 'to bind'). The concept of reliability emerged from the idea of being 'bound' or tied to one's commitments, making unreliable literally 'not bound' to trustworthiness.
The root of 'reliable' connects to binding and tying, suggesting that trustworthiness was originally understood as a physical constraint or bond. In our digital age, unreliable has expanded from describing people to describing everything from internet connections to data sources.
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