What exactly is 'weeping and gnashing of teeth'? I have always wondered.
There are several passages that refer to “weeping and gnashing of teeth.” In each of those passages (Matthew 8:12; 13:42,50; 22:13; 24:51; 25:30; Luke 13:28) that is to be the fate of those who are outside of heaven, in outer darkness, and/or in “the furnace of fire.” The weeping part is pretty clear. If God shall “wipe away all tears” (Isaiah 25:8; Revelation 7:17; Revelation 21:4) from those around his throne (the saved/the church), then it makes sense that those who are not among the saved and away from the presence of God would be weeping.
So, what is “gnashing of teeth?” There is a medical condition called “bruxism,” in which people grind their teeth, often while asleep. The name of that condition comes from the Greek word that is here translated as “gnashing.” Some scholars say that it is from a word that means “to bite” and describes the snarling of a wild animal as it attacks. The idea, then is that those who are apart from God attack each other and try to tear each other, much like a pack of hyenas fighting over a carcass. Since apart from God there is no good, then after the judgement the souls of the damned will, unsuccessfully, try to destroy each other; without goodness there is just hatred and envy. We can even see this to a certain extent today. Those who do not live by God’s principles bite and tear each other on the way to the top, in business or any other aspect of life. Those who live according to God’s way help others, rather than tearing them down.
I hope this helps in your understanding of the phrase.