Unreliable means you cannot depend on it. A car that starts four mornings out of five; a friend who says yes to everything and turns up to nothing; a website nobody has checked.
It stretches over people, machines and information alike: an unreliable witness, an unreliable memory, unreliable data, an unreliable bus service. The noun is unreliability and the adverb is unreliably. Weigh it before you use it — reliable is a real compliment about being steady, so unreliable is a real criticism, not a soft one. The best-known phrase is unreliable narrator, the term for a storyteller whose version of events you are not meant to trust. And watch the spelling: reliable ends in -able, so unreliable does too, never '-ible'.
1adjectiveunreliableThat cannot be depended on to be right, honest, or to work properly.
“The old washing machine is unreliable and floods the kitchen.”
synonyms: undependable, erratic
Unreliable is reliable with un- on the front, the small prefix that flips a word over: happy and unhappy, fair and unfair, kind and unkind. English adds it so freely that you can understand a brand new un- word the first time you hear one.
From un- + reliable.
Etymology from Wiktionary, CC BY-SA