arsen(o)-

Arsenic.

English arsenic, from Greek arsenikon, yellow orpiment (arsenic sulphide).

Arsenic is a semi-metallic element whose compounds are poisonous; the usual adjective is arsenical. Compounds in which arsenic has a valency of five are referred to by the adjective arsenic; those in which it has a valency of three are said to be arsenious. Arsenopyrite, also called mispickel, is the main ore of arsenic; arsenolite (Greek lithos, stone) is a naturally occurring form of arsenic oxide.

Arsenal—a collection of military equipment or a place where armaments are stored and made—is unconnected, originating in an Arabic term for a place of manufacture.