Jamieson, Fausset and Brown on Isaiah 30:33

33. Tophet--literally, "A place of abomination"; the valley of the sons of Hinnom, southeast of Jerusalem, where Israel offered human sacrifices to Moloch by fire; hence a place of burning (2Ki 23:10; Jr 7:31). Latterly Gehinnom or Gehenna, that is, valley of Hinnom, was the receptacle of the refuse of the city, to consume which fires were constantly burning. Hence it came to express hell, the place of torment. In the former sense it was a fit place to symbolize the funeral pyre of the Assyrian army (not that it actually perished there); the Hebrews did not burn, but buried their dead, but the heathen Assyrians are to be burnt as a mark of ignominy. In the latter sense Tophet is the receptacle "prepared for the devil (antitype to the king, Is 14:12-15) and his angels," and unbelieving men (Mt 5:22; 25:41; Mr 9:43, 44).