Old English carian, cearian be anxious, grieve; to feel concern or interest, from Proto-Germanic *karojanan (cf. Old High German charon to lament, Old Saxon karon to care, to sorrow), from the same source as care (n.).
Epsom salt is used as bath salts and for isolation tanks. Magnesium sulfate is the main preparation of intravenous magnesium. Internal uses include: Oral magnesium sulfate is commonly used as a saline laxative or osmotic purgative.
It's called a salt because of its chemical structure. The Epsom part is a place in England where it's found in natural springs. You can find it in most drugstores, usually around the aspirin and laxatives. Many grocery and natural food stores also carry it.