The phrase "(there shall be) weeping and gnashing of teeth" (in the original Greek ὁ κλαυθμὸς καὶ ὁ βρυγμὸς τῶν ὀδόντων) appears seven times in the New Testament as a description of the torments of the damned in Hell. It is thought to derive from a logion in the hypothetical Q source, which yielded Matthew 8:12, Matthew 13;42, Matthew 13:50, Matthew 22:13 and Luke 13:28. The other five occurrences are all within the context of parables and are widely held to be redactional additions by Matthew. Others, however, believe redactional theories of the parables are speculative at best, and offer little explanation as to the meaning of this phrase, and only speculate as to why Matthew and Luke included this apparently familiar saying.
The gnashing of teeth is literally the grinding of one's teeth together. To have one's teeth set on edge or to bite down in pain, anguish, or anger.
The phrase is also found as an idiomatic expression in colloquial English.