lost his marbles

If someone has lost his marbles, it means that he is in the process of going crazy in the mental sense, typically caused by stress of life. Losing your marbles is simply another way of saying someone seems to be losing it which refers to their sanity. It can be a mild statement about someone who seems to be overly stressed or more severe in the case of someone acting much different than they use to as a result of recent events in their life.