The Collaborative International Dictionary of English v.0.48:
Copperas \Cop"per*as\, n. [OE. coperose, F. couperose, fr.
(assumed?) L. cuprirosa, equiv. to G. cha`lkanqos, i. e.
copper flower, vitriol. See Copper and Rose.]
Green vitriol, or sulphate of iron; a green crystalline
substance, of an astringent taste, used in making ink, in
dyeing black, as a tonic in medicine, etc. It is made on a
large scale by the oxidation of iron pyrites. Called also
Note: The term copperas was formerly synonymous with vitriol,
and included the green, blue, and white vitriols, or
the sulphates of iron, copper, and zinc.
[1913 Webster] copper-bottom