archaic name for the Black Sea, from Latin Pontus Euxinus, from Greek Pontos Euxenios, literally "the hospitable sea," a euphemism for Pontos Axeinos, "the inhospitable sea." From eu- "good, well" (see eu-) + xenos "host; guest; stranger" (see xeno-).
According to Room, The Old Persian name for the sea was akhshaena, literally "dark," probably in reference to the sudden, dangerous storms that make the sea perilous to sailors and darken its face (or perhaps in reference to the color of the water, from the sea being deep and relatively lifeless), and the Greeks took this untranslated as Pontos Axeinos, which was interpeted as the similar-sounding Greek word axenos "inhospitable." Thus the modern English name could reflect the Old Persian one.