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κατὰ σκότον

From LSJ

Χαίρειν ἐπ' αἰσχροῖς οὐδέποτε χρὴ πράγμασιν → Non decet in rebus esse laetum turpibus → In schlimmer Not ist Freude niemals angebracht

Menander, Monostichoi, 544

Dutch (Woordenboekgrieks.nl)

in duisternis, in het geheim