Having all one's marbles is an expression that means being in full possession of one's mental faculties. The opposite of this phrase can be described as having a few screws loose, meaning mentally unstable or insane. Other antonyms for having all one's marbles include eccentric, deranged, demented, crazy, unhinged, addled, confused, perplexed, befuddled, bewildered, and disoriented. These antonyms signify a lack of mental clarity and stability, indicating that the individual is not in control of their thoughts and actions. Therefore, it is essential to maintain mental health to avoid being labeled with these antonyms.