What is the origin of the word buck as a nickname for a dollar bill?

Answer 1

The origin seems to be unknown - and be aware of the fact, that in earlier days it was a 1 dollar coin.
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Answer 2

The word buck—possibly an abbreviation from buckskin, an intrinsic "currency" for trade with American Indians known since 1746—However, two problems with this etymology are that a deerskin was worth more than a dollar, and that "buck", first attested in the 1850s, was quite rare until the twentieth century. But this is still the most likely explanation.

Answer 3

buck skins were used for trading.