A one-hundred dollor bill. C stands for "centum" the Latin word for 100. The word "note" actually come from the bill it's self. Which reads, "This note is leagal tender for all debts, public and private."
customer: how much does this skirt cost?
cashier: a c note
customer: huh?
cashier: one hundred dollars you moron you're gonna buy it or what?
customer: no you can keep it b!tch! a c note you gotta be out your d@mn mind!