Lewis Short
(adj.adj.) : Phlĕgĕthon, ontis, m., = Φλεγέθων (burning, blazing)
* A river in the Lower World, which ran with fire instead of water: Chaos et Phlegethon loca nocte silentia late,Verg. A. 6, 265: Tartareus,id. ib. 6, 551; Stat. Th. 4, 522: ardenti freto Phlegethon harenas igneus tostas agens,Sen. Thyest. 1018; id. Oedip. 162; id. Agam. 752. —Hence
* Phlĕgĕthontēus, a, um, adj., Phlegethontian: ripa,Claud. Rapt. Pros. 1, 88.
* Phlĕgĕthontis, ĭdis, f.adj., Phlegethontian: unda,Ov. M. 15, 532: lympha,id. ib. 5, 544.
Charlton T. Lewis, Charles Short, A Latin Dictionary