The word is entirely absent from Paul and the other NT writers, who prefer to speak in terms of the damned being destroyed. There is no word for “hell” in the Pauline epistles.
(Interesting, no? How could Paul write half the NT and say nothing about hell, when Jesus said so much? Maybe because we’re misunderstanding Jesus.)
The name means “Valley of Hinnom” or its full form “Valley of the son of Hinnom” … .
The valley was the scene of the idolatrous worship of the Canaanite gods Molech and Baal. This worship consisted of sacrificing children by passing them through a fire on Topheth (a high place) and into the hands of the gods (Jer 7:31; 19:4–5; 32:35). These practices were observed during monarchy at least under the reigns of Ahaz and Manasseh who themselves sacrificed their own children (2 Kgs 16:3; 21:6; 2 Chr 28:3; 33:6).
Josiah defiled the site as part of his reform program (2 Kgs 23:10; cf. vv 13–14), but the prophecy of Jeremiah indicates that it probably recurred later in the monarchy. Jeremiah prophesied that it would no longer be called Topheth or the Valley of Hinnom, but the valley of Slaughter because of the numerous Judeans killed and thrown into it by the Babylonians (Jer 7:29–34; 19:1–15). …
Other reasons given for the association are the fact that the Valley of Hinnom was noted for the fires of the Molech cult and later contained the continually burning fires of a refuse dump. Although Gehenna does not have these associations in the OT, the OT is the primary source of the association, particularly the prophecies of Jeremiah regarding the dead bodies of the wicked cast into Gehenna (7:29–34; 19:6–9; 32:35). The prophecies of Isaiah which ironically prophesy the threat of the Topheth readied for Molech himself (30:33), a devouring fire and everlasting burnings, and of a fire that will not be quenched readied for the wicked all contributed to the association (33:14; 66:24; cf. Isa 50:11; Dan 12:2; Mal 3:19).
Duane F. Watson, The Anchor Yale Bible Dictionary, 1992, 2, 926-927.
In short, gehenna refers to a garbage dump outside of Jerusalem, which is also the place where babies were burned alive as sacrifices to pagan gods. It’s a place of fire, burning, and suffering. But the garbage dump idea remains — at least as Jesus used the word.
(Mat 5:29-30 ESV) 29 If your right eye causes you to sin, tear it out and throw it away. For it is better that you lose one of your members than that your whole body be thrown into hell [gehenna= garbage dump]. 30 And if your right hand causes you to sin, cut it off and throw it away. For it is better that you lose one of your members than that your whole body go into hell [gehenna= garbage dump].
(Mat 18:9 ESV) 9 And if your eye causes you to sin, tear it out and throw it away. It is better for you to enter life with one eye than with two eyes to be thrown into the hell [garbage dump] of fire.
It makes perfect sense to compare God throwing someone into gehenna to throwing away an eye. “Throw” or “throw away” is the same Greek verb when used of eyes and of people.
The idea of gehenna isn’t eternal conscious torment so much as being burned up and consumed, like garbage. Whatever is tossed in the fire is destroyed.
(Mat 10:28 ESV) And do not fear those who kill the body but cannot kill the soul. Rather fear him who can destroy both soul and body in hell [garbage dump].
Why is God’s vengeance worse than a human’s? Humans can only kill you. God can not only kill you in this age, but he can then destroy you for all eternity — denying you the resurrection and punishing you for your sins.