"Be fastened" refers to the act of securing or attaching something in place. The antonyms for this word could be "detach," "loosen," or "unfasten." These words convey the opposite meaning of being locked in place and represent the act of freeing an object. Another antonym for "be fastened" could be "move," which implies the object was not locked down but now has more freedom of movement. Alternately, "broken" could be an antonym as it implies that the fastening process has been disrupted or the object is no longer secure. When considering the meaning of "be fastened," there are many antonyms that highlight the versatility of the English language.