He is mad as a hatter when he starts talking about conspiracy theories.
Он совершенно сумасшедший, когда начинает говорить о теориях заговора.
Definitions
The phrase 'mad as a hatter' refers to someone who is completely insane or eccentric. It originates from the 19th century when hat makers used mercury in the process of making felt hats, which often led to mercury poisoning and symptoms of madness.
