To become unfastened means to become detached or separated from its fastening. The antonym for this word would be to become fastened or secured. This can be achieved through various means such as tying, locking, or fastening with a clip or latch. Similarly, the opposite of becoming unfastened could be to tighten or reinforce the fastening. It is important to keep items properly fastened and secured to avoid any accidents or damages caused by things becoming unfastened. So, whether it's a door, window, or a piece of clothing, it is essential to ensure that it is properly fastened and secured.