word-forming element meaning "iron," used since late 18c. in medical terms and mineral names, from Greek sidēros "iron," which is of unknown origin. Beekes writes that "The Greeks got to know iron from Asia Minor, the Pontus and Caucasus, and it is likely that they took over the word for it from these areas as well." He points to resemblance to Udian (Caucasian) zido "iron."
He also mentions other theories: that the Greek word originally referred to meteoric iron, and is derived from Latin sidus "constellation;" that it refers to the redness of the metal and is related to sidē "pomegranate" or that it is connected to words for "silver" (such as Lithuanian sidābras).