There will be weeping there, and gnashing of teeth
Noun - Nominative Masculine Singular
Strong's Greek 1030: A grinding or gnashing. From brucho; a grating.
This noun appears 7 times. All were spoken by Jesus. All were used together with the word "weeping".
bruchó: to bite, to gnash
Original Word: βρύχω
Part of Speech: Verb
Phonetic Spelling: (broo'-kho)
Definition: to bite, to gnash
Usage: I grind or gnash, as with the teeth for rage or pain.
As verb, it appears exactly once in the stoning of Stephen:
Acts 7:51 “You stiff-necked people! Your hearts and ears are still uncircumcised. You are just like your ancestors: You always resist the Holy Spirit! 52Was there ever a prophet your ancestors did not persecute? They even killed those who predicted the coming of the Righteous One. And now you have betrayed and murdered him— 53you who have received the law that was given through angels but have not obeyed it.”
54When the members of the Sanhedrin heard this, they were furious and gnashed their teeth at him. 55But Stephen, full of the Holy Spirit, looked up to heaven and saw the glory of God, and Jesus standing at the right hand of God.
They were angry at Stephen's insults at them.
So weeping and gnashing express sadness and anger because of pain.