There was a time when the term bucks was only used to refer to the American dollar. Nowadays, the word is used to refer to money in general. Even the Indian rupee, in informal contexts, is being called a buck.
*Who is making the big bucks in your family?
The invention of paper money is a recent phenomenon. In the old days, when people wanted to buy something, they usually traded or bartered what they had. When a hunter wanted grain from a farmer, he usually traded animal skin or dried meat for it. The term ‘buck’ is actually the short form of ‘buckskin’. A buck, as you know is a male deer; ‘buckskin’, therefore, is the skin of this deer. Hunters, when they did business made use of ‘buckskins’. Later on, with the passage of time, the word ‘buckskin’ in everyday conversation was shortened to ‘buck’. When paper money was introduced much later, people began to call it ‘buck’.
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“Always end the name of your child with a vowel, so that when you yell, the name will carry.” — Bill Cosby