Definition of clam up

The origin of the phrase "clam up" can be traced back to the late 19th century, specifically to the American English dialect. “Clam” in this context has a meaning that is different from its literal definition as a type of shellfish. In slang, “clam up” describes someone who becomes unusually quiet, reticent, or uncommunicative. The etymology of the term “clam” in this context is uncertain, but some linguists believe it stems from the phrase "keep clammed," which was a regional variant of "keep mum" or "keep still". This phrase is a variant of "clam shell" or "clammediate," which were once used as slang for silence or tight-lippedness. The phrase “clam up” became widespread during the Gold Rush in California when there was an influx of new people from all parts of the world. It provided an apt description for those who became tight-lipped and uncommunicative, especially in situations involving secrets or sensitive information. The expression has since evolved into a popular idiom and used more generally to describe someone's sudden silences, particularly in social or professional settings.