The term "bills" for paper money comes from the earlier usage of "bill" as a written document. Originally, a bill referred to a sealed document, deriving from the Latin word "bulla," which meant a seal or a boss used to authenticate documents. Over time, the word bill came to mean various official documents, including banknotes or paper money, which were essentially documents promising exchange for real money or legal tender. So, bills are called bills because they are documents that serve as currency or a note for payment.

If the question refers to why the Buffalo Bills (football team) are called the Bills, that name honors "Buffalo Bill" Cody, a famous 19th-century American frontiersman and Buffalo hunter. The team name was chosen from a public contest reflecting the frontier theme connected to Buffalo's history.