A begging letter is a letter to a rich person or organisation, usually written by a poor person, or a person claiming to be poor, begging for money or help.
Examples of begging letters include a variant of the Nigerian 419 scam, where a letter is sent to a wealthy individual asking for financial assistance for orphaned children, emergency surgery, etc.
The May 1850 edition of Household Words contained an article entitled The Begging-Letter Writer written by the novelist Charles Dickens. In the article Dickens describes examples of the many begging letters he had received over the years, and the ruses employed by their writers to gain funds from the recipients.
Charities such as Children International tell child sponsors not to release their mailing addresses because of the potential for being a target of begging letters. Instead, sponsors use Children International as a go between.