To gnash is to grind your teeth together, usually because you are very angry or very unhappy — so unhappy it shows in your face and in your jaw. English almost always uses it with teeth: you gnash your teeth; people talk about the gnashing of teeth. You do not ‘gnash a problem’ in careful speech, though newspapers sometimes stretch it to mean ‘be furious about something’ without describing the mouth at all. The sound in the word is meant to feel a bit like the scrape of teeth. It is a strong, slightly old or literary word. In everyday talk, a child who is angry is more likely to ‘grit their teeth’ or just ‘be furious’. You still meet gnash in stories, in the Bible phrase ‘weeping and gnashing of teeth’ (great sorrow and rage), and in jokes about someone who has lost badly. There is a noun too, but the verb-plus-teeth is what learners need. The first letter is silent: it sounds like ‘nash’, not ‘guh-nash’.

咬牙、咬牙切齒（因氣憤或痛苦而磨牙）