In 1921, ranchers were dismayed at the sudden onset of lossage in their herds due to a strange condition: the animals bled to death. Small cuts failed to heal. The roughage cows eat will scratch their digestive systems, but unlike the normal case where such scratches are minor and readily heal, these scratches failed to heal and the animals died from internal hemorrhaging.
What seemed odd was that the animals were being fed hay from fields that appeared not dissimilar from that of previous years. No sudden invasive plants of a poisonous nature had been found.
A researcher by the name of Karl Paul Link, working under the aegis of the Wisconsin Alumni Research Fund (WARF), did a careful analysis of the ensilage from ranches that suffered losses and those that did not. He discovered that a chemical, dicoumorin, found in the ensilage of sweetclover hay from those ranches suffering the losses, was a powerful anticoagulant.
Dicoumarin is the result of a substance called coumarin, which is the chemical which gives new-mown hay its characteristic smell, being subjected to the heat and mold in a silo, and forming a double molecule. The year of the serious losses had been an unusually warm one after the ensilage was created.
This information is from Kingsley's book "Poisonous Plants", which is one of the first serious studies of the biology of plant toxins. Link named this substance after the organization that supported his research.
Hence the name. In the mid-1940s, this substance was used as a rodenticide. However, rat immunity to Warfarin appeared within ten years of its use as a rodenticide, and has forced the development of other "super-warfarins.
Warfarin was re-manufactured for human consumption, and used in the 1950s as an anticoagulant for victims of heart attacks and strokes, but gained fame when it was used to treat President Dwight D. Eisenhower after his 1956 coronary (while in office). Warfarin (also known under the brand names Coumadin, Jantoven, Marevan, Lawarin, Waran, and Warfant is an (blood thinner), and is essential for the prevention of and , the formation of blood clots in the blood vessels and their migration elsewhere in the body respectively.
Since Vitamin K is an important factor in the synthesis of clotting factors in the blood, it is considered an "antidote" for warfarin overdose.