The Collaborative International Dictionary
Knoppern
Knoppern \Knop"pern\, n. [Cf. G. knopper. See Knop.] (Zo["o]l.) A kind of gall produced by a gallfly on the cup of an acorn, -- used in tanning and dyeing.
Wiktionary
knoppern
n. A kind of gall produced by a gallfly on the cup of an acorn, and formerly used in tanning and dyeing.