If something is unusable, it is in such bad condition — or so badly made, so broken, so dirty — that you cannot use it at all. Not "hard to use", not "not very good": you simply cannot use it for its job. A phone that fell in the sea is unusable. A road buried under mud after a typhoon is unusable. A photo so dark you cannot see anyone's face is unusable. The word points at the thing, not at the person: it says the object has failed, not that you did something wrong.

It is a slightly formal, written-feeling word. In everyday talk people say "it's broken", "it doesn't work", "you can't use it any more". You will meet "unusable" in news reports, insurance letters, school notices, and repair shops — anywhere someone is telling you officially that a thing is finished. That formal feel is also why it sounds calm even when the news is bad.

One thing to notice: unusable does not always mean destroyed. A room can be unusable because the ceiling leaks; fix the leak and the room comes back. So the word describes the state a thing is in right now, and sometimes that state can change. People also stretch it a little to mean "useless for my purpose" — a teacher might call a student's data unusable when the numbers are all there but written so messily that nobody can read them.

Also: things can be unusable for one job and fine for another. Cracked rice is unusable for a nice bowl of white rice, but perfectly good for congee.