Okay. Let me clarify that I do not actually have any fluency with ancient Greek. But I do have a Liddell-Scott abridged lexicon, memories of conjugating verbs and declining nouns and adjectives in high school Latin lessons, and Wikipedia’s ancient Greek declension/conjugation tables.
So. The particular maxim we’ve got here is ηττω υπο δικαιου: êttô upo dikaiou.
The lexicon at the only entry for eta-tau-tau-anything refers me to eta-sigma-sigma-anything, and that’s all related to the verb meaning “to be less than, weaker than, inferior to another”, “to be beaten, to give way, to submit”, it’s even a legal term for “to lose one’s cause”. It may have been too long since high school Latin lessons, I can’t figure out the verb tense and voice, but I’m going to assume Oikonomides had a reason for translating this in the imperative.
upsilon-pi-omicron is a preposition. Its lexicon entry goes on for a solid half page, and what precisely it means depends on the case of the noun it goes with. Haven’t got that far yet; I’ll come back.
delta-iota-kappa-alpha-iota-omicron-upsilon okay looks like second declension noun in the genitive case, which means grammatical gender is indeterminate because of reasons, but English hasn’t got that outside pronouns anyway so none of you care. (It’s spelled wrong to refer to the Goddess Dikê anyway. I’m pretty sure.) So now I only have to comb through a quarter page of the lexicon to figure out that preposition! Might be “under” or “by” or “to” or “by reason of”, though, depending on precise context.
The noun “dikaios” (or “dikaia” or “dikaion”) might refer to justice or righteousness or equality or evenness or lawfulness or fairness or even moderation.
So I’m going to translate ηττω υπο δικαιου as “Submit to justice”: don’t set yourself against justice (or Justice), as [it, She] is stronger than you. (Oikonomides’s translation, “Be overcome by justice”, still reads more to me like, justice is a goal, work to get there.) It might equally well, if more loosely and verbosely translated, mean “When you lose a court case, accept the verdict”! But as you can see precisely how I got to either of those theres from here, you can translate it yourself how you choose.