Gehenna…means the Valley of Hinnom, a valley to the southwest of Jerusalem.
It was notorious as the place where Ahaz had introduced the fire worship of the heathen god Molech, to whom little children were burned (2 Chron 28:2-4).
Josiah stamped out that worship and ordered that the valley should be forever after an accursed place…it became the place where the refuse of Jerusalem was cast out and destroyed.
It was a kind of public incinerator. Always the fire smouldered in it, and a pall of thick smoke lay over it, and bred a loathsome kind of worm which was hard to kill (Mark 9:44-48).
So Gehenna, the Valley of Hinnom, became identified in people’s minds with all that was accursed and filthy, the place where useless and evil things were destroyed.