lose one's marbles

Then why are you standing there like a little boy who's lost his marbles?

Yes, we may be present at the creation of a very new world, and no, I have not lost my marbles.

"I actually did have one for a while a couple years ago, but he started to lose his marbles."

Now that they are under siege, it is no time for them to act as if they're losing their marbles.

He eyed me as if I had lost my marbles.

"Like I say, you've lost your marbles," he said again, but the bite was gone from his voice.

It is further proof that the old man has lost his marbles.

All three of them looked at me as if I'd lost my marbles.

The old boy had lost his marbles somewhere along the line.

Until now, I've felt like a very old man, losing all my marbles.