Usable means something that can be used or is capable of being used. Thus, un-usable means something that cannot be used or is incapacitated for use. Its antonyms are, therefore, words that connote the opposite meaning or imply that something is usable or functioning. The antonyms of un-usable include words such as operational, functional, working, practicable, viable, available, serviceable, workable, and useable. These terms denote the ability or ease of use and functionality of something. Unlike un-usable, they imply that an object is fit for use and can serve its desired purpose efficiently.