Easton's 1897 Bible Dictionary:

Hinnom
   a deep, narrow ravine separating Mount Zion from the so-called
   "Hill of Evil Counsel." It took its name from "some ancient
   hero, the son of Hinnom." It is first mentioned in Josh. 15:8.
   It had been the place where the idolatrous Jews burned their
   children alive to Moloch and Baal. A particular part of the
   valley was called Tophet, or the "fire-stove," where the
   children were burned. After the Exile, in order to show their
   abhorrence of the locality, the Jews made this valley the
   receptacle of the offal of the city, for the destruction of
   which a fire was, as is supposed, kept constantly burning there.

     The Jews associated with this valley these two ideas, (1) that
   of the sufferings of the victims that had there been sacrificed;
   and (2) that of filth and corruption. It became thus to the
   popular mind a symbol of the abode of the wicked hereafter. It
   came to signify hell as the place of the wicked. "It might be
   shown by infinite examples that the Jews expressed hell, or the
   place of the damned, by this word. The word Gehenna [the Greek
   contraction of Hinnom] was never used in the time of Christ in
   any other sense than to denote the place of future punishment."
   About this fact there can be no question. In this sense the word
   is used eleven times in our Lord's discourses (Matt. 23:33; Luke
   12:5; Matt. 5:22, etc.).


Hitchcock's Bible Names Dictionary (late 1800's):

Hinnom, there they are; their riches