In November 1905, the famed American author Mark Twain was ill and depressed. His wife Olivia had recently passed away. He was not in the mood to humor a cheeky salesman named J. H. Todd, who had mailed Twain some promotional materials for his snake oil medicine. 

This remedy, which Todd had given the slightly aggrandized name, the Elixir of Life, supposedly had the power to cure meningitis and diphtheria—diseases that had killed two of Twain’s children.

LOC

Well, you might say the letter just hit Twain the wrong way. He immediately dictated an angry response. When his wife was alive, she knew Twain’s habit of writing angry letters. Realizing these missives might be what is now called a “career limiting move,” Olivia surreptitiously removed letters from the mailbox before the postman picked them up.

Without Olivia to remove the letters, this communique went to the intended recipient.

Nov. 20. 1905

J. H. Todd
1212 Webster St.
San Francisco, Cal.

Dear Sir,

Your letter is an insoluble puzzle to me. The handwriting is good and exhibits considerable character, and there are even traces of intelligence in what you say, yet the letter and the accompanying advertisements profess to be the work of the same hand. The person who wrote the advertisements is without doubt the most ignorant person now alive on the planet; also without doubt he is an idiot, an idiot of the 33rd degree, and scion of an ancestral procession of idiots stretching back to the Missing Link. It puzzles me to make out how the same hand could have constructed your letter and your advertisements. Puzzles fret me, puzzles annoy me, puzzles exasperate me; and always, for a moment, they arouse in me an unkind state of mind toward the person who has puzzled me. A few moments from now my resentment will have faded and passed and I shall probably even be praying for you; but while there is yet time I hasten to wish that you may take a dose of your own poison by mistake, and enter swiftly into the damnation which you and all other patent medicine assassins have so remorselessly earned and do so richly deserve.

Adieu, adieu, adieu!
Mark Twain

“An idiot of the 33rd degree” might be one of the most satisfying insults in modern letters!