The antonyms for the phrase "fastened on" include released, let go, freed, disengaged, and detached. These words signify the opposite of being connected or fixed on something. When something is fastened on, it is secured or attached to something else. However, when something is released or let go, it is no longer bound or restricted. Similarly, when something is detached, it is separated or disconnected. All these antonyms serve to describe a state of being unbound, free or unhinged from something.