δ212Δελφῖνα νήχεσθαι διδάσκεις
you are teaching a dolphin to swim.
[sc. A proverbial phrase] in reference to those who train someone in matters in which they are already expert. Also [sc. attested is] "you are tying a dolphin by the tail"; in reference to to the impossible; because of…
Δελφῖνα νήχεσθαι διδάσκεις· ἐπὶ τῶν ἐν ἐκείνοις τινὰ παιδοτριβούντων, ἐν οἷς ἤσκηνται. καὶ Δελφῖνα πρὸς τὸ οὐραῖον δεῖς· ἐπὶ τῶν ἀδυνάτων· διὰ τὸ εὐκίνητον καὶ ὀλισθηρόν.