Why do they call it "hard money"?

It is difficult to find an answer to this question. Plenty of speculation has been heard. Some people say that it's because the money is used for "hard to do" loans. Others say it is because the loans are "hard to get" or "hard to pay." Some believe that it is called hard money because traditionally it has been "real money" in the sense that it is not borrowed. Institutions loan borrowed money and in this sense, they loan "soft money."