The phrase "
Happy as a
clam" makes a little more sense when you hear the full
expression: "happy as a clam at
high tide." The clam is supposed to be happy at high
tide because it is
safe then from land animals and people digging for it (but come
low tide, watch out!) This
idiom is supposed to date from American colonial times.
Sources:
http://www.bartleby.com/81/7945.html
http://www.erickraft.com/markdorset/topicalguide/happyasaclam.html
http://nersp.nerdc.ufl.edu/~nickym/Cliches.html