Gehenna is an English transliteration of the Greek word γέεννα, which in turn is from the Hebrew word gê’ hinnom, literally the valley of Hinnom.
John F. Walvoord writes,
"All the references to gehenna, except James 3:6, are from the lips of Christ himself, and there is an obvious emphasis on the punishment for the wicked after death as being everlasting. The term gehenna is derived from the Valley of Hinnom, traditionally considered by the Jews the place of the final punishment of the ungodly. Located just south of Jerusalem, it is referred to in Joshua 15:8 and 18:16, where this valley was considered a boundary between the tribes of Judah and Benjamin. In this place human sacrifices were offered to Molech; these altars were destroyed by Josiah (2 Kings 23:10). The valley was later declared to be 'the valley of slaughter' by Jeremiah (Jer. 7:30-33). The valley was used as a burial place for criminals and for burning garbage. Whatever its historical and geographic meaning, its usage in the New Testament is clearly a reference to the everlasting state of the wicked, and this seems to be the thought in every instance. In James 3:6 the damage accomplished by an uncontrolled tongue is compared to a fire which 'corrupts the whole person, sets the whole course of his life on fire, and is itself set on fire by hell.' "Christ warned that a person who declares others a fool 'will be in danger of the fire of hell' (Matt. 5:22). In Matthew 5:29 Christ states that it is better to lose an eye than to be thrown into gehenna, with a similar thought regarding it being better to lose a hand than to go into gehenna (Matt. 5:30). In Matthew 10:28 believers in Christ are told not to be afraid of those who kill the body, but rather to 'fear him which is able to destroy both soul and body in hell' (KJV). A similar thought is mentioned in Matthew 18:9, where it is declared better 'to enter life with one eye than to have two eyes and be thrown into the fire of hell.' In Matthew 23:15 Christ denounces the Pharisees who 'travel over land and sea to win a single convert, and when he becomes one, you make him twice as much a son of hell as you are.' In Matthew 23:33 he denounces the Pharisees and the scribes, asking the question, 'How will you escape being condemned to hell?' In Mark 9:43, 45, 47, the thought recorded in Matthew about it being better to lose part of the body than to be cast into hell is repeated (cf. Matt. 5:22, 29, 30). Luke 12:5 contains a similar thought to that expressed in Matthew 10:28, that one should fear the devil far more than those who might kill them physically. Though not always expressly stated, the implication is that the punishment will have duration and be endless." John F. Walvoord in Four Views on Hell, p. 20
William Crockett writes,
"In the New Testament the final destination of the wicked is pictured as a place of blazing sulfur, where the burning smoke ascends forever. This would have been an effective image because sulfur fires were part of life for those who lived in the Jerusalem of Bible times. Southwest of the city was the Valley of Hinnom, an area that had a long history of desecration. The steep gorge was once used to burn children in sacrifice to the Ammonite god Molech (2 Kings 23:10; Jer. 7:31; 32:35). Jeremiah denounced such practices by saying that Hinnom Valley would become the valley of God's judgment, a place of slaughter (Jer. 7:32; 19:5-7). As the years passed, a sense of foreboding hung over the valley. People began to burn their garbage and offal there, using sulfur, the flammable substance we now use in matches and gunpowder. Eventually, the Hebrew name ge-hinnom (canyon of Hinnom) evolved into geenna (gehenna), the familiar Greek word for hell (Matt. 5:22, 29; 10:28; 18:9; 23:33; Mark 9:43, 45; Luke 12:5). Thus when the Jews talked about punishment in the next life, what better image could they use than the smoldering valley they called gehenna? "In the intertestamental period, gehenna was widely used as a metaphor for hell, the place of eternal damnation. Later, in rabbinic literature, we find gehenna given a location—in the depths of the earth, and sometimes in Africa beyond the Mountains of Darkness. Some Jews, of course, took the fiery images literally, supposing that Hinnom Valley itself would become the place of hellfire and judgment (1 Enoch 27:1-2; 54:1-6; 56:3-4; 90:26-28; 4 Ezra 7:36). But this view was minor and not widely held in Judaism. The New Testament also rejects this view, saying that gehenna is already in some sense prepared elsewhere (Matt. 25:41), just as heaven is (Matt. 25:34; John 14:2; Heb. 11:16)." William Crockett in Four Views on Hell, p. 58