Buck

No one knows for sure why a US dollar is called a buck, but there are several theories. One is that it comes from the game of poker during the 19th century when a silver dollar was used to replace the buckhorn knife that was passed from player to player to indicate the next dealer. Another plausible theory is that buck is short for buckskin, which was a common medium of exchange in the early pioneering days. As early as 1748, there is written evidence of trading whiskey for buckskins, as follows, “Every cask of whiskey shall be sold to you [Native Americans] for five bucks.” The transition from bucks meaning buckskins to dollars seems quite reasonable. See also Buck up and Pass the buck.