The weeping and gnashing of teeth refer to the greatest possible suffering, conveyed in the Greek word klauthmos, which is the sadness felt when suffering a loss (Matthew 2:18, Acts 20:37). Those in the darkness gnash their teeth in powerless fury and despair, because their suffering will never end (see Daniel 12:2; Matthew 3:12, Matthew 18:8, Matthew 25:46)—primarily a spiritual, rather than a physical, anguish.
The weeping and gnashing of teeth serves to indicate the greatest conceivable form of suffering. The Greek word klauthmos expresses that sadness experienced when suffering a loss (Matthew 2:18; Acts 20:37). The gnashing of teeth indicates powerlessness and despair combined with fury and anger. The prospect these people face is, after all, a never-ending suffering (see Daniel 12:2, Matthew 3:12, Matthew 18:8, Matthew 25:46).1
Weeping is an expression of their sadness, while the gnashing of teeth points to perpetual suffering. These expressions indicate the pain of hell, which is first and foremost a spiritual as opposed to a physical form of suffering.2
11 Or, je vous déclare que plusieurs viendront de l'orient et de l'occident, et seront à table avec Abraham, Isaac et Jacob, dans le royaume des cieux.