As happy as a clam
Very happy and content.
Why would clams be happy? It has been suggested that open clams give the appearance of smiling. The derivation is more likely to come from the fuller version of the phrase, now rarely heard - 'as happy as a clam at high water'. Hide tide is when clams are free from the attentions of predators; surely the happiest of times in the bivalve mollusc world. The phrase originated in the north-eastern states of the USA in the early 19th century. The earliest citation that I can find is from a frontier memoir The Harpe's Head - A Legend of Kentucky, 1833:
"It never occurred to him to be discontented... He was as happy as a clam."
The first record that I can find of the 'high water' version is from the Pennsylvania newspaper The Adams Sentinel, August 1844:
"Crispin was soon hammering and whistling away as happy as a clam at high water."
The expression was well-enough known in the USA by the late 1840s for it to have been included in John Russell Bartlett's Dictionary Of Americanisms - A Glossary of Words And Phrases Usually Regarded As Peculiar To The United States, 1848:
"As happy as a clam at high water," is a very common expression in those parts of the coast of New England where clams are found.
Also in 1848, the Southern Literary Messenger from Richmond, Virginia expressed the opinion that the phrase "is familiar to everyone".
See other 'as x as y similes'.
See other phrases that were coined in the USA.