Where does the saying "happy as a clam" come from?

It comes from “happy as a clam at high tide,” at time in which they are nestled nicely in the mud, away from predators and scavenging birds. The exact origins are unknown, but there are citations from the 1840s (in the U.S.) that reference it as being widely know.

It may have caught public attention because the shape of a clam suggests a wide smile.

i always thought it was because they have those big wrap-around grins (the crack between each shell).

I have never in my life seen a pissed off clam, so I assume all clams are happy, thusly, happy as a clam I am.

@Sueanne_Tremendous I’ve had a no so happy clam spit at me right after i dug it up, ha ha!