A “C-note” is slang for a $100 bill. “C” is the Roman numeral for one hundred; “note” means Federal Reserve Note. “Laid his ‘C’ note right on the old line” was cited in print in August 1929, in a newspaper story by Damon Runyon (1880-1946). Runyon used underworld slang in his stories and probably popularized the slang term.
 
The Roman numerals “V” (for a $5 bill) and “X” (for a $10 bill), and so on, had been used in the 19th century.
 
[This entry was prepared with the assistance of Bill Mullins and Garson O’Toole of the ADS-L, the listserv of the American Dialect Society.]
     
 
Wiktionary: c-note
Noun
c-note (plural c-notes)
1.(US, informal) A one-hundred dollar banknote.
2.(music) the lowest note of an instrument, written below the staff and the D note.
 
(Random House Historical Dictionary of American Slang, A-G)
C-note n. [C + NOTE] Gamb. a one-hundred-dollar bill.
1930 Liberty (Oct. 11) 30: We gave him five C notes and two tens.
1954 Schulberg Waterfront 8. He was always good for fifties and C-notes peeled off the fat roll.
 
9 May 1837, The Herald (New York, NY), pg. 2, col. 3:
V’s, *X*’s, L’s, and C’s of the Dry Dock Bank.
 
2 August 1929, Springfield (MA) Republican, “Canzoneri-Mandell In Chicago Ring Tonight” by Damon Runyon, pg. 29, col. 1:
My operatives inform me that a commission has been receive in this town from Mr. Al Well, matchmaker of the Queensboro arena, in New York, who is betting $100 against $266, and taking Canzoneri for his.  Moreover, Mr. Well, more familiarly known as Weskit, has laid his “C” note right on the old line.
 
Google News Archive
1 April 1938, The Daily Times (Beaver County, PA), pg. 11, cols. 6-7:
Dictionary Co-Editor
Gives Ruling On Poor
English Or Slang
CHICAGO, Apr. 1—“Any cabbage head should know you can’t catch the larks by being in cahoot with hoodlums even for a C-note cash on the nail every day because there is always the danger of passing in your checks unexpectedly.
 
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10 July 1955, Lewiston (ID) Morning Tribune, “2 Well-Heeled Gamblers Free; Cash Missing” (AP), pg. 7, col. 3:
Wilson said he had been “saving C-notes for couple of years.”
 
Google News Archive
2 March 1956, Eugene (OR) Register-Guard, “Oil Lobbyist Neff Flutters No C-Notes at Senators” by Frederick C. Othman, pg. 12A, col. 6:
WASHINGTON—I’ve been keeping an eye on John C. Neff, the Nebraska oil lobbyist who likes to pass out $100 bills to deserving polticians, and I must report that he wears flaps on his pockets.
 
Not a single C-note fluttered by mistake to the Senate caucus room as the stubby Mr. Neff walked glumly to the hot seat of the senators investigating his penchant for slipping hundred dollars Williams to right-thinking politicians.