τεΐν
From LSJ
Τὸ νικᾶν αὐτὸν αὑτὸν πασῶν νικῶν πρώτη τε καὶ ἀρίστη. Τὸ δὲ ἡττᾶσθαι αὐτὸν ὑφ' ἑαυτοῦ πάντων αἴσχιστόν τε ἅμα καὶ κάκιστον. → Τo conquer yourself is the first and best victory of all, while to be conquered by yourself is of all the most shameful as well as evil
English (LSJ)
[ῑ], Dor. and Ep. dat. sg. of σύ (q.v.).
German (Pape)
[Seite 1079] dor. dat. sing. zu σύ, = σοί, auch ep., wie Hom. Il. 11, 201 Od. 4, 629. 829 u. sonst; im Orakel bei Her. 5, 60. 61. S. σύ.