Something that is unusable cannot be used, especially because it is broken or not safe.

The normal supply of water has turned brown and unusable. 日常用水变成褐色的了，不能用了。

The virus deletes files and corrupts essential core microchips, leaving computers unusable. 这种病毒删除文件，损坏关键的核心微芯片，造成电脑瘫痪。