Clam Up Share definition Flag To suddenly keep unusually quiet, often in regards to the divulging of information. Carol clammed up about the incident at the bakery, so we never did find out whatever happened to Nate. Clam Up by Diggity Monkeez April 30, 2005 0 0 Get merch Get the Clam Up mug. Buy Now — $32.95 See more merch