Where did this saying come from:?

There's an old story that the expression derives from Dr. Samuel Mudd, who unwisely took pity on Abraham Lincoln's assassin, John Wilkes Booth. Mudd treated the broken ankle Booth suffered in his leap to the stage of Ford's Theater; for his trouble, he was sentenced to life in a federal prison. But Mudd isn't being commemorated in "his name is mud." The phrase first appeared in print in 1820, 45 years before Lincoln's assassination. It probably originates in another obscure bit of English slang -- "mud" was an eighteenth century equivalent of our "dope" or "dolt" and was used through the nineteenth century by union workers as a rough equivalent of "scab."

John Wilkes Booth broke his leg while escaping after shooting Abraham Lincoln in 1865. He was given medical help by Dr Samuel Mudd, who didn't then know about the assassination. Mudd was convicted of being Booth's conspirator, although he is widely believed to have been innocent. Actually, whether Mudd was innocent or not is of little consequence in regard to the origin of this phrase. It was in general circulation before Lincoln was assassinated. This comes from John Badcock's (aka 'J. Bee’) Slang - A dictionary of the turf etc., 1823:

"Mud - a stupid twaddling fellow. ‘And his name is mud!’ ejaculated upon the conclusion of a silly oration, or of a leader in the Courier."

The phrase appears to be one of the many that, when a news story arises, match the gist of the story and later become associated with it.