The word "un-fasten" refers to the action of opening or releasing a fastened item. Its antonyms, on the other hand, are words that convey the opposite meaning. These include "fasten," "close," "secure," "lock," "button-up," "zip-up," "tie," "connect," and "join." For example, instead of "un-fastening" a seatbelt, you would "fasten" it. If you were to "close" a door, you would be doing the opposite of "un-fastening" it. Similarly, "secure" would be an antonym if you were talking about locking a door or safe. Other antonyms of "un-fasten" depend on the context and item being fastened, but they all relate to its opposition.