A C-note is a one-hundred-dollar banknote, in informal American English. C is the Roman numeral for 100, and old US hundred-dollar bills had that C printed on them; note here means the piece of paper money. People have used the slang since the 1920s. You might hear it in films, crime stories, or from older American speakers: “He pulled out a C-note.” It is not a polite bank word, it is not used for other countries’ money, and it is a little dated now. In ordinary talk, people are more likely to say a hundred dollars or a hundred-dollar bill.