Definition
To behave in a strange, silly, or eccentric manner, suggesting a lack of mental clarity or being slightly crazy.
脑袋有喜鸭（形容行为愚蠢或精神失常）
Ever since he started talking to the air by himself, I've thought he has bats in the belfry.
自从他开始自言自语后，我就觉得他精神有些失常。
Her illogical decision made me think she has bats in the belfry.
她做出的不合理的决定让我觉得她精神有些失常。
It was clear that he had bats in the belfry when he tried to comb his hair with a fork.
当他试图用叉子梳头时，很明显他精神有些失常。