Ἀχερόντειος
From LSJ
Ἢ λέγε τι σιγῆς κρεῖττον ἢ σιγὴν ἔχε → Sile, melius vel loquere silentio → Was besser ist als Schweigen, sage oder schweig
English (LSJ)
α, ον,
A of Acheron, ναῦς Call.Hec.31.3:—also Ἀχερόντιος, E.Alc.443 (lyr.), Ar.Ra.471:—fem. Ἀχεροντιάς, άδος, νύξ AP5.240 (Paul. Sil.): and Ἀχερούσιος, α, ον (also ος, ον A.Ag.1160), Th.1.46:—fem. Ἀχερουσιάς, άδος, X.An.6.2.2, Pl.Phd.113a.