From Longman Dictionary of Contemporary Englishgnash your teethgnash your teethANGRYto be very angry or unhappy about something, or to move your teeth against each other so that they make a noise, especially because you are unhappy or angry → gnash
Examples from the Corpus
gnash your teeth• He kept baring and gnashing his teeth. 21.• But he has spent three or four years out in the darkness, gnashing his teeth.• He laid her on the kang, wailing and gnashing his teeth.• Their heads thrash about on the bloodied floor, gnashing their teeth and foaming at the mouth.• Gacbler and his colleagues would often be stymied by some problem, gnashing their teeth and getting nowhere.• An unlikely assassin was left-winger Jason Wilcox, a youth team star while Dalglish was still gnashing his teeth at Anfield.• Then he threw up his hands and wailed and gnashed his teeth, for the world had already touched his father. 