Charon (Gr.) The Egyptian Khu-en-ua, the hawk-headed Steersman of the boat conveying the Souls across the black waters that separate life from death. Charon, the Sun of Erebus and Nox, is a variant of Khu en-ua. The dead were obliged to pay an obolus, a small piece of money, o this grim ferryman of the Styx and Acheron; therefore the ancients always placed a coin under the tongue of the deceased. This custom has been preserved in our own times, for most of the lower classes in Russia place coppers in the coffin under the head of the dead for post mortem expenses.
Source: H.P.Blavatsky - The Theosophical Glossary