Clam Up

Etymology:

• This idiom was first used by a writer. According to an observation made by the writer, he thought that the two halves of clamshell were actually like the lips of a person. Due to any reason a clam can shut tightly and appear silent or lifeless. This action of a clamshell was resembled to closed lips of a person, who was not willing to say or speak about anything. There are a few other idioms and expressions that are used in the same manner. But it depends on the writing style and choice of words.