gnash one's teeth
Fig. to grind or bite noisily with one's teeth. Bill clenched his fists and gnashed his teeth in anger. The wolf gnashed its teeth and chased after the deer.
gnash your teeth
to show you are angry or annoyed about something bad that you cannot do anything to stop His advisers are gnashing their teeth in frustration because he refuses to attack his opponent on foreign policy issues.
Usage notes: also used in the form gnashing of teeth: The first test-tube baby was born in 1978, to considerable gnashing of teeth.