Δελφῖνα νήχεσθαι διδάσκεις: ἐπὶ τῶν ἐν ἐκείνοις τινὰ παιδοτριβούντων, ἐν οἷς ἤσκηται → Teaching dolphins to swim: is applied to those who are teaching something among people who are already well versed in it
ἰθύδικος, -ον (Α)
δίκαιος, χρηστός.
[ΕΤΥΜΟΛ. < ἰθύς (Ι) + -δικος (< δίκη), πρβλ. αυτόδικος, φυγόδικος].