early 15c., "black antimony, antimony sulfide" (a powder used medicinally and in alchemy), from Old French antimoine and directly from Medieval Latin antimonium (11c.), a word of obscure origin.
Probably it is a Latinization of later Greek stimmi "powdered antimony, black antimony" (a cosmetic used to paint the eyelids), from an Arabic source (such as al 'othmud), unless the Arabic word is from the Greek and the Latin is from Arabic (which would explain the a- as the Arabic direct article al-). Probably it is ultimately from Egyptian stm "powdered antimony;" the substance was used there as a cosmetic from at least 3000 B.C.E.
In French, by folk etymology, it became anti-moine "monk's bane." As the name of a brittle metallic element in a pure form, it is attested in English from 1788. Its chemical symbol Sb is for Stibium, the Latin name for "black antimony," which word also was used in English for black antimony. Related: Antimonial; antimoniac.