1670s as the name of a black mineral, oxide of manganese (used from ancient times in glassmaking for removing coloring matter), from French manganèse (16c.), from Italian manganese, alteration or corruption of Medieval Latin magnesia (see magnesia). From 1783 in English as the name of the metallic element.
From the middle of the eighteenth century the combinations of manganese were studied by various chemists, and finally, in 1774, the metal manganese was isolated by Gahn, but for years there was much confusion in regard to its specific name, and it was not until after the beginning of the present century that the name manganese (mangan in German) began to be generally adopted. [Century Dictionary, 1897]
Related: Mangano-, manganic, manganous, manganate.