Actually, the word "clam" is short for "clamshell" which is another slang
term for a buck, a simoleon or a dollar.
I suppose you now want to know why the word "clamshell" means a smacker.
At one time, clamshells, in the form of many small ones strung together,
were a currency used by native Americans, mostly in California.
This DeAnza College quizola key page mentions them.
"Clamshell made into 'strings' of money and these strings were the standard
"The Miwok" chapter from "Handbook of Indians of California" by A. L. Kroeber
(1919) goes into valuation details.
"Clamshell disk currency was less precious than in the north, though that may
have been one of the directions from which it reached the Miwok. Its value in
American terms is said to have averaged $5 a yard, only a fraction of the
figure at which the southern Maidu rated it. Whole strung olivella shells went
at $1 a yard among both groups."
The Phoebe A. Hearst Museum of Anthropology web site has some nice pictures
in their "Money and Wealth" section.
"Clam shell beads, strings"
Clamshells are used as currency still to this day, but by members of the
Yapese culture, and not in the Americas, according to this Yap Art Studio &
Gallery web page.
"Shell money is used as payments with stone money to purchase canoes, land,
or pay fines."
If you need any clarification, please feel free to ask.
Google search on: slang clam dollar
Google search on: clamshell money OR currency
Looking Forward, denco-ga - Google Answers Researcher