An Etymological Dictionary of the German Language/knirschen

by Friedrich Kluge

knirschen, vb., ‘to gnash,’ MidHG. *knirsen, may be inferred from knirsunge, f., ‘gnashing,’ and zerknürsen, ‘to crush, squash’; for sch from s after r comp. Hirsch and Arsch; comp. ModDu. knarsen, knersen, ‘to gnash, crash,’ knarsetanden, ‘to gnash with the teeth.’