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Being happy; feeling delighted.
This phrase apparently has two versions. The full version is "as happy as a clam at high tide [or water]," and then there's the short version "as happy as a clam." To really grasp the origins of this expression, one needs to understand why a clam would be 'happy' in the first place, and the reason is actually highlighted in the fuller version.

Basically, clams are most vulnerable when the tides are low because that's the time when humans can easily dig them out of the ground. In higher waters, however, clams are far more difficult to find and dig up. Hence, a clam is happiest during a high tide because it means they are less likely to be caught and eaten!

An American poet, John G Saxe, uses the short version of the phrase in the Sonnet to a Clam, from 1840:

"Inglorious friend! most confident I am
Thy life is one of very little ease;
Albeit men mock thee with their similes,
And prate of being 'happy as a clam!'"

The full version of the saying is in use at around the same time, as seen in the Bangor Daily Whig And Courier from 1841:

"Your correspondent has given an interesting, and, undoubtedly correct explanation of the expression: 'As
happy as a clam at high water.' His pursuits must be anything but Clam-berous, if we may judge from his
knowledge of the nature and habit of this interesting little fish."

Reference: newspaperarchive, worldwidewords
Note: The definitions for sayings can be found with ease, but finding the origins of phrases proves far more challenging. Looking back through history, it's tough to find the place or person in which a phrase has its roots. We are limited to what can be found in writings, such as books, poems, newspapers, and plays. Often times, phrases will be quoted from century old newspapers or books.

The popular idioms that are quoted, however, are likely already commonly known from their respective times, and thus, are probably older. For instance, an idiom may show up in a newspaper from 1850s, but that does not mean the idiom originated from that newspaper. What that does tell you though, is that the phrase was being used over 150 years ago.
* Having had nothing to eat for most of the day, I was as happy as a clam when I came home from work and my wife had prepared a feast for dinner!

* Dennis was as happy as a clam in high water when the dentist fixed the tooth pain he was experiencing all week long.
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