[have] a screw (loose|missing)

to behave in a crazy and uncontrollable manner

The origin of the idiom "have a screw loose" is not definitively known, but it is believed to have originated from the mechanical world, where a loose or missing screw could cause a machine or device to malfunction. It can be used to highlight eccentricities, irrational thinking, or unusual actions, often in a light-hearted or humorous manner, though it should be used with caution to avoid causing offense.

After witnessing her erratic behavior at the party, it became clear that Sarah had a screw loose.
Her constant mood swings, impulsive decisions, and inconsistent behavior suggest that she has many screws loose.
His constant forgetfulness, disorganized thoughts, and impulsive actions suggest that he has some loose screws.
Jane's sudden outbursts of laughter at inappropriate times suggest that she might have a screw missing.
John's conspiracy theories and outlandish beliefs make me think he has a few screws loose.