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.

2009Guardian The uninhibited heroine loses her marbles and ends up doing herself unspeakable harm before being put out of her misery by her sensitive writer boyfriend.