"Gehenna" is the name of the lowland area near Jerusalim, where children were killed and burned as idolatrous sacrifices to the pagan god Moloch. 

         ("...And they set up the heights of Topheth in the valley of the sons of Ennomus, to burn their sons and their daughters in fire, which I did not command and which did not come to My heart." Jer.7:31)

         Later, during the time of King Josiah, the valley of Gehenna was transformed into a dump for smoldering city garbage, so the word became a synonym for the "dump" of the sinful history of mankind.

 "Gehenna" symbolizes the human experience of "unquenchable", unbearably painful, spiritual, moral, psychological, physical suffering that befalls him as a consequence of rejection, violation of the Will of God, as a fruit of his iniquities.