The word "un-fastened" refers to something that is not securely or firmly attached. The antonyms for this word would be "fastened", "secured", "fixed", "attached", and "affixed". These words refer to things that are firmly or securely in place, whether it be a door, a latch, a zipper or a button. In addition to these antonyms, words such as "locked", "bolted", "clasped", and "buttoned" could also be used to describe items that are securely fastened. Overall, there are many antonyms for "un-fastened" that convey the sense of something being securely attached or locked in place.