The "Continental" was the nickname for the first series of
United States banknotes, issued by the authority of the
Continental Congress to finance the
American Revolution in the late 1700's. Continental dollars were printed and
counterfeited in such bulk that they quickly became almost worthless, much like the
reichsmark in the
Weimar Republic of
Germany. Most people at the time used money printed by
state governments or by private banks: incidentally, a similar practice arose in the
Confederate States of America during the
Civil War.
The rapid depreciation of the Continental's value gave rise to the expression "not worth a Continental," i.e. not worth anything.