What is the origin of the idiom "happy as a clam" and when to use it?

The origin of the phrase "happy as a clam" can be traced back to early 19th-century America. The original form of the phrase was "happy as a clam at high water," with "high water" referring to the highest point of the tide. The phrase likely originated due to the association of clams being safe and content during high tide, as they are protected from being harvested. This idiom is used in various contexts to describe a person's state of extreme happiness or contentment. It is typically employed in colloquial and informal language, often in a lighthearted or playful manner.