lose your marbles

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!
See also: lose, marble
Farlex Dictionary of Idioms.

lose your marbles

INFORMAL
If you lose your marbles, you become crazy. At 83 I haven't lost my marbles and my memory is, thank God, as clear as it ever was. People are talking about him as if he's lost his marbles. Note: You can also say that someone has all their marbles, meaning that they are not crazy. He's in his eighties but he clearly still has all his marbles.
See also: lose, marble
Collins COBUILD Idioms Dictionary, 3rd ed.

lose your marbles

go insane; become irrational or senile. informal
Marbles as a term for ‘a person's mental faculties’ probably originated as early 20th-century American slang. The underlying reference is apparently to the children's game played with multicoloured glass balls.
1998 Spectator At least, that is how I recall the event, but I am losing my marbles.
See also: lose, marble
Farlex Partner Idioms Dictionary

lose your ˈmarbles

(informal) become crazy or mentally confused: They say the old man has lost his marbles because of the strange things he’s been saying, but I’m not so sure.
See also: lose, marble
Farlex Partner Idioms Dictionary
See also:
  • lose all one’s marbles
  • lose marbles
  • lose one’s marbles
  • lose (one's) marbles
  • lose one's marbles
  • lose one's buttons
  • lose (one's) buttons
  • lose one's mind
  • lose your mind
  • lose (one's) mind
References in periodicals archive
Three new casual, broad-appeal games, "Reversi", "Lose Your Marbles" and "Blackjack", would also be made available exclusively to SmarTone customers in Hong Kong for a limited period of time.
-- "Lose Your Marbles" Rolling, rolling, rolling...marbles keep on rolling.