metallotherapy (n.)

an undecomposable elementary substance having certain recognizable qualities (opacity, conductivity, plasticity, high specific gravity, etc.), mid-13c., from Old French metal "metal; material, substance, stuff" (12c.), from Latin metallum "metal, mineral; mine, quarry," from Greek metallon "metal, ore" (senses found only in post-classical texts, via the notion of "what is got by mining"); originally "mine, quarry-pit," probably a back-formation from metalleuein "to mine, to quarry," a word of unknown origin. Perhaps related somehow to metallan "to seek after," but Beekes finds this "hardly convincing."

The concept was based on the metals known from antiquity: gold, silver, copper, iron, lead, and tin. As an adjective, "of or covered with metal," from late 14c. As short for heavy metal (rock music) by 1980. Metal-work "work, especially artistic work, in metal" is by 1724.

1846, "medical treatment of disease," from Modern Latin therapia, from Greek therapeia "curing, healing, service done to the sick; a waiting on, service," from therapeuein "to cure, treat medically," literally "attend, do service, take care of" (see therapeutic).