Definition of Phlegethontic in US English:

Phlegethontic

adjective

rare, poetic
  • Of, relating to, or resembling the mythical fiery river Phlegethon; specifically blazing, burning.

Origin

Early 17th century; earliest use found in Cyril Tourneur (d. 1626), writer and soldier. From classical Latin Phlegethont-, Phlegethōn + -ic. Compare post-classical Latin Phlegethonticus.