No one knows for sure exactly who first used the phrase "happy as a clam," but experts generally think that it got its start in the United States along the East Coast. Evidence suggests that a longer version of the saying was circulating prior to the early- to mid-19th century. The meaning is thought to come from the way clams are protected during high tide, and although the connotations have changed slightly, this original concept is still is the heart of the expression today.
This phrase is actually a part of the bigger phrase "happy as a clam in high water" or "at high tide." Historians aren't sure when this longer saying first appeared, but they generally agree that the shortened one was in use starting around 1830. In 1840, John G. Saxe used the truncated version in quotations in his poem, Sonnet to a Clam, suggesting that he was repeating something that already was commonly said. Eight years later, the magazine, The Southern Literary Messenger declared that using the full version wasn't necessary anymore, because everyone was familiar with the short form. It was also included in the 1848 version of John Russell Bartlett's Dictionary Of Americanisms: A Glossary of Words And Phrases Usually Regarded As Peculiar To The United States.
Source of Origin
Experts typically claim that this saying is of clear American origins. More specifically, they attribute it to those living on the East Coast, particularly in the New England region. Clams are plentiful in this area, so the people who lived there prior to the mid-19th century likely would have been very familiar with them.
Anyone who has ever hunted for clams knows that they must be dug when the tide is low. They’re almost impossible to find in high tide, and it would be dangerous to venture too far out into deep water. People say these creatures are happy at high tide, therefore, because they're in no danger, at least from humans, of being made into a meal.
Applied to people, this phrase means that someone feels safe or secure. Things are well in the world and danger is, for the moment, at a safe distance. A symbolic clamshell — perhaps made of good friends and family, financial security and similar items — contains the person well, providing protection with its virtually uncrackable hardness. As Saxe writes in his last line of his sonnet: “thy case is shocking hard!”
Much of the original meaning for this phrase has been retained over time. When a person uses it, the idea is still that they are content. Some of the connotations regarding protection from danger have been lost, however, largely because this association comes from the last half of the longer version of the saying, which people rarely use anymore. Most people who use this expression don't really believe they are in physical trouble and are merely trying to say that they feel safe from harm in a much more general sense.
Most of the time, when people use this idiom, they do so during periods of excitement, or when things are going especially well. These good times are not completely sustainable, however, and everyone has bad days or rough periods in their lives. As a result, occasionally, people who have endured tragedy say they were in a bubble or shell of happiness, which sad events shattered. This is an allusion to the original "happy as a clam" phrase, but it implies that the period of joy and protection is over.