ι

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

Plato, Laws, 626e

Frisk Etymological English

ί
Grammatical information: pcle
Meaning: added particle, demonstrative-augmentative, ὁδ-ί, οὑτοσ-ί, νυν-ί etc. (Att.; also El. το-ΐ, Boeot. ταν-ί etc.), rarely -ίν.
Origin: IE [Indo-European] [285] *-i demonstr. pcle
Etymology: To be compared are the added enclitics Skt. and GAv. īm, ī; further -i in Hitt. aši, eni-, uni- the one mentioned, he, in Lat. utī etc.; Gothic has -ei. See Schwyzer 611 n. 3.