Why is the dollar bill called a "buck"?

According to The Atlantic Monthly, the term buck most likely originated from the use of buck skins or buck hides during colonial times as a commodity of exchange. They were also a loose measure of value between the colonists and Native Americans.

The Atlantic Monthly provides information from Mitford M. Matthews' book, "A Dictionary of Americanisms" that describes a situation involving a man who was robbed of the value of 300 bucks in 1748. The term goes unlisted in many early slang dictionaries and apparently did not gain national popularity until the 1890s, a good example of how slowly slang terms were disseminated before the days of radio and television.