Seonji is blood of animal, especially cattle, which is cooled and hardened. It is used as an ingredient for soup or stew. Cattle's seonji and dried radish leaves are added to the sagolguk (sagol soup, or boiled beef leg bones), in order to make the seonjiguk (seonji soup). Pigs' seonji is become an ingredient for sundae (Korean Sausage).
Seonji has been derived from "Senji" in Manchu, which means blood.
References
Seonji Wikipedia(Text) CC BY-SA