If you say that someone is gnashing their teeth, you mean they are angry or frustrated about something. If you couldn't attend either of the concerts and are currently gnashing your teeth at having missed out, don't despair. gnash your teeth to 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 (Probably from the sound)
[ 'nash ] (transitive verb.) 15th century. From Middle English gnasten. Origins unkown, but presumably Germanic. Compare Old Norse gnastan and German knirschen.