Why Is The Dollar Sign A Letter S?

The letter S appears nowhere in the word "dollar", yet an S with a line through it ($) is unmistakably the dollar sign. But why an S? Why isn't the dollar sign something like a Đ (like the former South Vietnamese đồng, or the totally-not-a-joke-currency Dogecoin)? » 2/08/15 11:29am 2/08/15 11:29am