Alan Wachtel: 

Have you ever heard the origin of the expression "happy as a clam," or wondered why a clam should be thought notably happy? Like "speak of the devil," it's a truncation of a longer expression. In his commonplace book, Animal, Vegetable, Mineral, published in 1972, Louis Kronenberger says: 

 
"I wish I had made up a word that had entered the language; the most I can claim is to have dredged up a metaphor that was subsequently decapitated. It was a metaphor I found listed somewhere and had never seen in print, whereupon I used it several times in a magazine with a large circulation -- 'happy as a clam at high tide.' Thereafter I began to see it in print and to hear it in speech in the truncated form 'happy as a clam.' Thus what gave it point it had been robbed of: 'happy as a clam' is neither good sense nor good nonsense." 

John Ciardi in A Second Browser's Dictionary (1983) does not cite Kronenberger but attributes the full expression "happy as a clam at high tide" to colonial times, and explains that "the water then was too high for clamming. It is simple enough to suppose that the happiest one can make a clam is by leaving it alone." He goes on: 

 
"As a random inquiry into the nature of idiom, I have asked hundreds of people what they supposed could make a clam happy. Very few, in fact almost none, knew of the earlier and lengthier form, yet all understood the clipped form accurately [as meaning very happy]." 

I think there are some lessons here about both clams and people. 

Here is a misunderstanding of what makes a clam happy, from Edouard Manet, writing to Zacharie Astruc, during the summer of 1879, when he was spending time at a hydrotherapy spa in the Parisian suburb of Bellevue, undergoing treatment for syphilis: 

 
"As you say so well, time is a great healer. Consequently, I'm counting heavily on it, living like a clam in the sun when there is any, and as much as possible in the open air; but even so, the country has charms only for those who are not obliged to stay there." 

 

(quoted by Otto Friedrich in Olympia: Paris in the Age of Manet, page 283) 

A Topical Guide to the Complete Peter Leroy (so far) is a work of fiction. The characters, incidents, dialogues, settings, and businesses portrayed in it are products of the author’s imagination and are not to be construed as real. Any resemblance to actual events or persons, living or dead, is entirely coincidental. 

All rights reserved. No part of this guide may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the author. 

Portions of A Topical Guide to the Complete Peter Leroy (so far) were first published by Voyager, Inc., as part of The Complete Peter Leroy (so far).

The illustration at the top of the page is an adaptation of an illustration by Stewart Rouse that first appeared on the cover of the August 1931 issue of Modern Mechanics and Inventions. The boy at the controls of the aerocycle doesn’t particularly resemble Peter Leroy—except, perhaps, for the smile.