The term 'Happy as a ___' can be completed with the answers 'CLAM' and 'LARK'. The phrase 'happy as a CLAM' is an idiom that means to be content and satisfied. It is thought to have originated in the 1800s, and is likely derived from the fact that clams are sedentary creatures that don't have to worry about predators or other dangers. The phrase 'happy as a LARK' is also an idiom that means to be joyous and cheerful. This phrase is thought to have originated in the 1600s, and is likely derived from the fact that larks are known for their beautiful singing.