σὺν δόλῳ

From LSJ

ὁ γὰρ μανθάνων κιθαρίζειν κιθαρίζων μανθάνει κιθαρίζειν → he who is learning the harp, learns the harp by harping

Source

Greek > English (Woodhouse)

craftily, by craft, by treachery, in an underhand way