"As happy as a clam"

“As happy as a clam” is an American phrase that puzzles many, who ask if and why clams are happy. “He was as happy as a clam” was cited in print in 1833. “I should be happy as a clam at high water, as the sailors say” was cited in print in 1836. In “high water” or “high tide,” clams are said to be “happy” because they stand less of a chance of being collected and eaten.

The Phrase Finder
As happy as a clam
Meaning
Very happy and content.
Origin
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.”

World Wide Words
HAPPY AS A CLAM
(...)
The saying is very definitely American, hardly known elsewhere. The fact is, we’ve lost its second half, which makes everything clear. The full expression is happy as a clam at high tide or happy as a clam at high water. Clam digging has to be done at low tide, when you stand a chance of finding them and extracting them. At high water, clams are comfortably covered in water and so able to feed, comparatively at ease and free of the risk that some hunter will rip them untimely from their sandy berths. I guess that’s a good enough definition of happy.

The saying in its shortened form is first recorded in the 1830s, though it is almost certainly a lot older; by 1848 the Southern Literary Messenger of Richmond, Virginia could say that the expression in its short form “is familiar to every one”.

(Oxford English Dictionary)
phr. as happy as a clam, etc.: well pleased, quite contented. U.S. colloq.
1834 Harvardiana I. 121 That peculiar degree of satisfaction, usually denoted by the phrase ‘as happy as a clam’.
1844 ‘J. Slick’ High Life N.Y. I. xi. 179 They seemed as happy as clams in high water.
1848 J. R. Bartlett Dict. Americanisms (at cited word), ‘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.
1873 J. H. Beadle Undeveloped West 799 A thousand or more negroes thronged the streets ‘happy as clams at high tide’.

Google Books
The Harpe’s Head:
A Legend of Kentucky
By James Hall
Philadelphia, PA: Key & Biddle
1833
Pg. 46:
He was as happy as a clam.

5 October 1833, Saturday Morning Transcript (Boston, MA), pg. 19, col. 5:
A GOOD REASON. A man being overtaken by a shower, sought shelter from the rain in the house of a negro fiddler. On entering, he found the negro in the only dry spot in the house—the chimney corner—as happy as a claim, fiddling most merrily.
(...)
Dunstable Telegraph.

14 January 1836, Newark (NJ) Daily Advertiser, pg. 2, col. 1:
Dear Mrs. Butternut, I must leave off, for I can’t say any more, only that if I was once more safe at home, I should be happy as a clam at high water, as the sailors say.

21 February 1840, New Bedford (MA) Mercury, “Jonathan Slick’s First Visit to the Park Theatre,” pg. 4, col. 2:
They seemed as happy as clams at high-water; ...

Google Books
Dictionary of Americanisms:
A glossary of words and phrases, usually regarded as peculiar to the United States
By John Russell Bartlett
New York, NY: Bartlett and Welford
1848
Pg. 81:
CLAM. The popular name of a very common shell-fish. “As happy as a clam at high water,” is a very common expression in those parts o the coast of New England where clams are found.

Google Books
September 1856, United States Magazine, “Clam Philosophy” by Jaques Maurice, pg. 280, col. 1:
How melancholy, then, is the compliment to human nature when we say, “As happy as a clam!” Happy! He is never happy, even at “high-tide.”

OCLC WorldCat record
Happy as a clam
Author: Nancy Tucker
Publisher: Bloomfield, CT : Collie-Flower Records, cp 1989.
Edition/Format: Musical cassette : Cassette recording : Songs : English

OCLC WorldCat record
Happy as a clam : and 9,999 other similes
Author: Larry Wright
Publisher: New York : Prentice Hall General Reference, ©1994.
Edition/Format: Book : English : 1st ed