The Japanese term sennin is a loanword from Middle Chinese SenNyin 仙人 "immortal person", known also as xian "immortal; transcendent; genie; mage; djinn; sage; hermit" in Daoism.
Sennin is a common Japanese character name. For example, Ikkaku Sennin (一角仙人 "One-horned Immortal") was a Noh play by Komparu Zenchiku (金春禅竹, 1405–1471). The Japanese legend of Gama Sennin (蝦蟇仙人 "Toad Immortal") is based upon Chinese Liu Hai (劉海), a fabled 10th-century alchemist who learned the secret of immortality from the Chan Chu ("Three-legged Money Toad").