What is origin of the word dun used as in a letter demanding payment debt?

The word "dun" used in the context of demanding payment for a debt originates from the Old English word "dyngan," meaning "to push, shove, or thrust."

The term likely evolved from the practice of creditors physically pushing or shoving debtors to remind them of their obligations.

Here's how it came to be associated with demanding payment:

1. Physical Prompting: In the past, creditors would literally "push" or "shove" debtors to remind them of their debts, perhaps by pushing them towards their homes or towards the creditor's office.

2. Figurative Usage: Over time, the act of "pushing" became a figurative representation of persistently demanding payment.

3. Evolution of Meaning: The word "dyngan" evolved into "dun," and the meaning shifted from physical pushing to a more abstract sense of insistent pressure or harassment.

Therefore, "dun" in this context is a metaphorical representation of the relentless pursuit of debt repayment.