informal to start acting in a strange way and forgetting things.
I may be old, but I haven't lost my marbles yet.
lose (one's) marbles
To be or become mentally deficient, incompetent, or deranged; to become of unsound mind. My poor grandmother started losing her marbles after she had a stroke.I've been so sleep deprived lately that it feels like I've lost my marbles!
lose (all) one's marbles
and lose one's mind
Fig. to go crazy; to go out of one's mind. What a silly thing to say! Have you lost your marbles?Look at Sally jumping up and down and screaming. Is she losing all her marbles?I can't seem to remember anything. I think I'm losing my mind.