Definition & Meaning of "(as|) (close|dumb) as an oyster"

What is the origin of the idiom "close as an oyster" and when to use it?

The idiom "close as an oyster" originated from Shakespeare's play "The Merry Wives of Windsor" and refers to someone who is uncommunicative, secretive, or reluctant to share information. In the play, the character Pistol says, "Why, then the world's mine oyster, which I with sword will open." The metaphor compares the closed and secretive nature of an oyster's shell to a person who is tight-lipped or unresponsive. Over time, the idiom evolved to describe someone who is unresponsive or lacking in intelligence.