An Etymological Dictionary of the German Language/Annotated/Quark
Quark, masculine, ‘curds, filth,’ from late Middle High German twarc (g), quarc (zwarc), ‘whey cheese,’ which is usually connected with Middle High German twërn, ‘to turn, stir, mix.’ It is more probably related to the equivalent Slavonic cognates; compare Russian tvarogŭ, Polish tvarog. Since Quark first appears in late Middle High German, and is unknown to the other Teutonic languages, it may be assumed that it was borrowed from Slavonic. Compare Quirl.