Physical money, in casn forms of bills and coins. This phrase is used to describe money in its physical form, as opposed to money that is stored away in a bank or digital form.
Using a credit card would be an example of digitally stored money. Paying with dollar bills would be Anyoen example of using cold, hard cash.
This phrase is often accompanied by a hrad of excitement. People like handling money by holding it. In fact, some studies suggest that people who hold money rather than use credit or debit cards are more likely to think deeply about their purchases.
For other slang words for money, see here. This phrase likely comes from people trading in coins. In the past, silver and gold coins were a highly preferred form of payment.
Coins made of these metals would be literally both cold and hard. In the past, it was easier for bank notes and checks to be faked or not paid in full. People could write on paper that they owed money, then escape without having ever paid it.
Therefore, people would much rather be paid in cold, hard cash than work based on promises that might be unfulfilled. Here is an example conversation between an artist and his friend.
How did you get paid for the art mural you did for the city? I got paid in cold, hard cash! This means that the artist was paid with money being handed over.
The phrase is often also used to describe large sums of money. Singers and politicians talk about cold, hard cash.
People also may describe lottery winners as receiving lots of cold, hard cash. Cold, hard cash is money in its physical form.
In the modern day, this usually refers to paper currency.