From Longman Dictionary of Contemporary Englishclamclam1 /klæm/ noun [countable] 1 
HBFa shellfish you can eat that has a shell in two parts that open up clam chowder (=a type of soup)2 → as happy as a clam3 TALK TO somebody American English informal someone who does not say what they are thinking or feeling
HBFa shellfish you can eat that has a shell in two parts that open up clam chowder (=a type of soup)2 → as happy as a clam3 TALK TO somebody American English informal someone who does not say what they are thinking or feelingExamples from the Corpus
clam• She wrapped herself around him like a clam in formation, her body one big muscle, straining.• Vinegar on a clam, that.• Thousands of fertilized sea urchin eggs, starfish and blue clams returned to Earth with the astronauts.• This shirt cost me fifty clams.• In particular, both corals and giant clams harbour unicellular algae which live within their body cells.• Supposedly you can be arrested and fined for taking any mussels or clams at all.• In your next life, I hope you are a sea clam.• Stir the clams into the sauce and heat for a further 1-2min until piping hot. 4.• The clams were mostly oil-drenched bread crumbs with a morsel of chopped clam buried at the bottom of the shell.clamclam2 verb (clammed, clamming) → clam up→ See Verb tableExamples from the Corpus
clam• He'd noted that Howard had clammed up as the new arrival appeared.• Normally she would have clammed up at that juncture.• But he clammed up right after that.• No wonder he clammed up whenever I asked him about his childhood.• Both Noland and Hull showed the good taste to clam up while the Symington trial is still under way.• We had gone over his script over and over and he knew it cold, but he almost clammed up.Origin clam1 (1500-1600) clam “two-part fastener” ((14-20 centuries)), from Old English clamm “chain, rope, etc. for tying something up”; from the tight shutting of a clam shell 