History of the word copperas
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The Brockhaus Konversationslexikon (Brockhaus Encyclopedia, 14th edition, 1894-1894)

explains the German term Kupferwasser (copper water) as 


  veraltete Bezeichnung für unreinen (kupferhaltigen) Eisenvitriol


which means outdated name for impure (copper-containing) iron vitriol.

Note that iron vitriol is an old term for iron(II) sulfate,  such as vitriolic oil is an outdated name for sulfuric acid.

If you take a look at English wikipedia page on iron(II)sulfate, you'll see that one of the common minerals is a heptahydrate, melanterite. If you follow the link, you will recognize that one of the associated minerals is chalcanthite, which is $\ce{CuSO4·5H2O}$.

To cut the long story short, the term copperas stems from the fact that natural iron(II)sulfate was often found to be associated and maybe contaminated with copper(II)sulfate.