Becoming unfastened means to become unattached or loose. There are several antonyms for becoming unfastened such as securing, tightening, fastening, fixing, and attaching. Securing refers to making something safe and stable by attaching it firmly to another object. Tightening means to make something firm or taut by applying pressure. Fastening refers to joining two objects together through a device such as a buckle or clamp. Fixing means repairing or making something stable by attaching it firmly. Attaching refers to connecting one object to another by means of an adhesive or mechanical device. All of these antonyms convey the opposite meaning of becoming unfastened.