In 1980, at the depth of the last bear market in bonds, the 10-year was yielding around 16%. This meant that in general, if you borrowed money at that time, you would be paying more than 18% per annum on the loan (anyone who lent you money instead of the lending to the US Government by buying a Treasury, would be expecting a much higher rate of return for the greater risk).
So, if you’d borrowed $100,000 in 1980, you’d need to pay at least $18K to finance the loan per year (for simplicity’s sake, I’m not bothering to include principal repayments).