Why Is a Hundred Dollar Bill Called a C Note?
A "C note" is a slang term for a hundred dollar bill that came into being due to the Roman numeral for one hundred being C. The letter C was also printed on $100 bills.
The term was widely used in the 1920s and 1930s as well as in several gangster movies. C note may also refer to "century," as a century is 100 years. While C note is still used by some to refer to a $100 bill, it is now far more common to hear it referred to as a "Benjamin," as of 2014, due to Benjamin Franklin's picture on the $100 bill.