In Mark 9:18, this verb describes one of the symptoms afflicting the demon-possessed boy—teeth grinding, often associated with seizures or extreme distress. The physical manifestation points to the spiritual bondage tormenting the child. The grinding of teeth appears elsewhere in Scripture as an expression of rage or anguish (as when people gnash their teeth at Stephen), but here it's part of a medical-spiritual crisis that Jesus' disciples cannot resolve. The term captures the violent, involuntary nature of the boy's suffering.