One of the mysteries of the English language finally explained.
Treated as plural. Fish of inferior quality, especially those of a species considered too coarse or ill-tasting for general consumption, or unsuitable for sportfishing, and hence formerly caught or bought chiefly by the poor; sometimes opposed to game fish. Also in singular: a fish of this sort.
Mid 18th century (in an earlier sense). From rough + fish. With the specific use in quot. 1763 at sense 1 compare earlier ruffe.
rough fish/ˈrʌf ˌfɪʃ/
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