The idiom "噤若寒蝉" is formed by combining four characters: "噤" meaning "to keep silent", "若" meaning "like", "寒" meaning "cold" or "winter", and "蝉" meaning "cicada". These characters are arranged in a simile structure where "噤若" translates to "silent as", and "寒蝉" refers to a "winter cicada", an insect known for being inactive and silent in cold weather, thus collectively conveying the meaning of someone being too afraid or cautious to speak.