"Lost marbles" is a phrase that refers to someone who has become mentally unbalanced or has lost their sanity. There are several synonyms that can be used to convey the same meaning, including "crazy," "insane," "mad," "nuts," "bonkers," "loony," "deranged," "demented," "unhinged," and "psychotic." These terms all describe someone who is experiencing a break from reality or is behaving in an irrational or unpredictable manner. While some of these words may be used colloquially or as insults, it is important to remember that mental health is a serious matter that should be treated with empathy and respect.