Happy as a clam is an idiomatic phrase, used to refer to someone content, without anything to inhibit their joy.
The full form of the expression goes as “happy as a clam at high tide” rooting in the fact that clams are the safest during a high tide, as they are the most protected from predators at that time.
“Happy as a clam” originates from the north-eastern United States, in the early 1800’s.
One of the first citations of the phrase appeared in the 1833 frontier memoir, titled “A Harpe’s Head – A Legend of Kentucky”.
The expression started spreading already in the 19th century, emerging in a variety of sources in the US by the 1840’s.
The 20th century saw the phrase becoming a globally recognized expression.
“Happy as a clam” was first defined on Urban Dictionary in 2007.