A valley near Jerusalem, where children were made to “pass through the fire to Moloch.” Josiah threw dead bodies, ordure, and other unclean things there, to prevent all further application of the place to religious use. (2 Kings xxiii. 10, 11.) Here Sennacherib's army was destroyed. (Isaiah xxx. 31-33.) The valley was also called “Gehinnom” (valley of Hinnom), corrupted into Gehenna; and Rabbi Kimchi tells us that a perpetual fire was kept burning in it to consume the dead bodies, bones, filth, and ordure deposited there.
(Hebrew, toph, a drum. When children were offered to Moloch, their shrieks were drowned by beat of drum.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Tophet from Infoplease: