Piltdown \PILT-doun\ , noun;
1. (a) Related to an alleged fossil skull which was alleged to be found at Piltdown in Sussex, England in 1912
1. (b) The supposed primitive hominid to which the remains were ascribed; as Piltdown hoax, Piltdown jaw, Piltdown skull
1. (c) Piltdown man: The primitive hominid thought to be represented by the Piltdown skull, named Eoanthropus dawsoni and regarded by some as the missing link
2. Figurative and extended use: Suggestive of Piltdown man or the Piltdown hoax; primitive, outdated
I can't find the origin of Piltdown, but the story of the Piltdown man is probably more interesting anyway. Basically, in 1912 fragments of a skull and jawbone were collected from a gravel pit at Piltdown in East Sussex England. They were reconstructed and given a Latin name Eoanthropus dawsoni, the second portion given in honor of the man who discovered them, Charles Dawson. Many heralded them as the missing link between humans and the apes. 16 years later they unveiled a memorial to mark the site of the groundbreaking discovery, which still stands today.
Fast forward to 1953 when Time Magazine published evidence gathered by three scientists that the Piltdown man's skull was actually composed of three distinct species: A medieval age human skull, a 500-year-old Sarawak orangutan jaw, and chimpanzee fossil teeth. Plus, it had been artificially aged with an iron solution and chromic acid, and someone filed the teeth to give them a shape that was more in line with a human diet.
So why did it work? Well, apparently it played right into the assumptions of the day. Scientists then believed that a large modern brain preceded the modern omnivore diet, and the Piltdown skull proved exactly that. Plus, Europeans liked the idea that the earliest humans were in Eurasia so they didn't exactly take a critical eye to the discovery.
Who exactly forged the skull is unknown and, as one would expect, the details of its discovery are not well documented. Its acceptance set early research on human evolution back decades and was a waste of time (and paper) for the people who worked on the estimated 250+ papers written on the topic. Today the hoax serves as a reminder to the scientific community of the importance of being thorough and critical of any new discovery, and to keep prejudices out of the scientific forum.
A couple resources: TalkOrigins, Wikipedia