copperas
noun
cop·per·as ˈkä-p(ə-)rəs 
: ferrous sulfate
Word History
Etymology
Middle English coperas, from Anglo-French coperose, from Medieval Latin cuprosa, probably from aqua cuprosa, literally, copper water, from Late Latin cuprum
First Known Use
14th century, in the meaning defined above
Medical Definition
copperas
noun
cop·per·as ˈkäp-(ə-)rəs
: a green hydrated ferrous sulfate FeSO4·7H2O used especially in making inks and pigments —used especially of commercial grades not to be used internally