Unusable means something cannot be used for its intended purpose. When your bicycle has a flat tire and a broken chain, it's unusable until you fix it. When a computer crashes and won't turn on, it becomes unusable. A smashed phone screen might make the device unusable if you can't see or touch anything properly.

Something can be unusable temporarily or permanently. A flooded soccer field is unusable until the water drains and the grass dries out. But a board game with missing pieces might be permanently unusable because you can't play properly without them.

The word often describes things that should work but don't, whether due to damage, poor design, or circumstances. A dictionary written in a language you don't understand would be unusable to you specifically, even though it works fine for readers who know that language. When software has so many bugs that it keeps freezing, programmers say it's unusable. A dull knife is unusable for cutting vegetables safely.

People also use unusable to describe information or ideas that can't be applied in a practical way. If a teacher's instructions are so confusing that nobody can follow them, students might say those directions are unusable.