An Etymological Dictionary of the German Language/Quark
Quark, m., ‘curds, filth,’ from late MidHG. twarc (g), quarc (zwarc), ‘whey cheese,’ which is usually connected with MidHG. twërn, ‘to turn, stir, mix.’ It is more probably related to the equiv. Slav. cognates; comp. Russ. tvarogŭ, Pol. tvarog. Since Quark first appears in late MidHG., and is unknown to the other Teut. languages, it may be assumed that it was borrowed from Slav. Comp. Quirl.