Undependable means not reliable or trustworthy. When something or someone is undependable, you can't count on them to do what they're supposed to do or what they said they would do.

An undependable friend might promise to meet you at the park but often forgets or shows up an hour late. An undependable alarm clock might work fine for three days, then fail to go off on the morning of your big presentation. When your science partner is undependable, you end up doing most of the project yourself because you can't trust them to finish their part.

The word applies to things and people. A car becomes undependable when it keeps breaking down unexpectedly. Weather can be undependable when forecasts keep changing. A student who turns in homework only sometimes, regardless of what they promise, earns a reputation for being undependable.

Being undependable affects how others see you. If you're consistently undependable, people stop asking you to help with important tasks or including you in their plans. They can't risk counting on you. The opposite is dependable or reliable: someone or something you can count on to come through when needed.