To lose one’s marbles

To behave strangely or silly, to be out of one’s mind

You can use this phrase in ………… language.

 Spoken
Informal

Example 1
Have you lost your marbles? You can’t go outside just in your pyjamas, it’s freezing!

Example 2
Our granny just turned 90, but still hasn’t lost her marbles. She even attends some lectures at university!