knirschen
From LSJ
κάλλιστον ἐφόδιον τῷ γήρᾳ ἡ παιδεία (Aristotle, quoted by Diogenes Laertius 5.21) → the finest provision for old age is education
knirschen, facere stridorem. – mit den Zähnen, dentibus frendere od. infrendere; dentibus stridere. – Knirschen, das, der Zähne, stridor dentium.