The word "un-fastens" means to undo, unbutton, or unclasp. Some of the antonyms for un-fastens include attaches, connects, fastens, fixes, snaps, and secures. Attaches refer to combining or affixing two things together, while connects mean linking or joining two objects. Fastens denote the action of keeping something firmly in place, and fixes indicate the process of mending, repairing, or stabilizing something. Similarly, snaps mean closing or shutting with a click, while secures refer to making something safe and protected from danger. Overall, the antonyms for un-fastens directly indicate the opposite actions of disrupting, releasing, or opening.