ἀδελφιδοῦς
From LSJ
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
German (Pape)
[Seite 32] ὁ, Neffe, Bruder- od. Schwestersohn, 'Thuc. u. Folgde.
French (Bailly abrégé)
v. ἀδελφιδέος.
Russian (Dvoretsky)
ἀδελφιδοῦς: стяж. к ἀδελφιδεός.
Lexicon Thucydideum
filius fratris vel sororis, son of a brother or sister, 2.101.6. 4.101.5.