A bruxer habitually and involuntarily grinds their teeth. You may be one of these, since it’s been estimated that about one in four people do so. There’s continuing controversy in the dental profession over the extent to which it is a natural action rather than a condition needing treatment; many animals regularly grind their teeth as a way of sharpening them (a process called thegosis) and some experts argue that a proportion of human tooth-grinding may be an evolutionary hangover from this.
But many bruxers grind their teeth excessively, often through stress or nervous tension, usually while they’re asleep so they aren’t aware they’re doing it. This is bad, as it wears away tooth enamel, can cause cracks in teeth, and often leads to sore and tired facial and jaw muscles. One way to resolve the problem is to make a plastic guard, sometimes called a bruxing appliance, to relieve the symptoms at night.
The term is dental surgeons’ jargon, more often found in the US than in the UK; dictionaries generally don’t include it, though the base term for the concept, bruxism (where brux rhymes with flux), has been around since the 1930s and is well established. It’s modern Greek, derived from brukhein, to gnash the teeth.