There are lots of other words for money in the US and elsewhere. Cash, mullah, .... In any case, the term greenback refers to the color that all US bills had up until the time I was in elementary school. They were all a greenish color. Paper money is actually a fabric -- not paper. It is made from cotton and other materials so that it lasts longer and is harder for the chumps to print in their basements. That's also why most greenbacks aren't green today. Chump, by the way, is another insulting word (if you didn't already know).