"craftsman who makes wooden vessels," attested from late 12c. as a surname, either from Old English (unattested) or from a Low German source akin to Middle Dutch cuper, East Frisian kuper, from Low German kupe (German Kufe) "cask," cognate with Medieval Latin cupa (see coop (n.)).
A dry cooper makes casks, etc., to hold dry goods, a wet cooper those to contain liquids, a white cooper pails, tubs, and the like for domestic or dairy use. [OED]The surname Cowper (pronounced "cooper") preserves a 15c. spelling.