Intro: I may have gone a little overboard with the explanation on this one. I hope somebody some time finds it useful, at least!
(Gi wo mite sezaru wa yuu naki nari;
“To see what is right and not do it is to lack courage.”)
This one is pretty self-explanatory. Knowing what you (as a human being) should do (as a matter of human morality) and yet not doing it means that you lack courage. In a more relativistic phrasing, believing that you’re supposed to do something, but failing to do it, means that you lack the courage of your convictions.
義 (gi) is a complicated noun. It can refer to morality, justice, honor, Buddhist teachings, a word’s meaning (as in 同義語 – dougigo – “synonym”), or even an in-law relationship. It can also tie into duty, as in 義務 (gimu, “obligation, responsibility”) or 義理 (giri, “sense of duty, honor”). The object-marker を (wo) marks it and connects it to the verb 見る (miru, “to see”), here in conjunctive form. This verb is paired with another one, する (suru, “to do”), in a formal or old-fashioned negative form.
Brief aside here: the verb we think of as する was originally merely 為 (su); the ru comes from one of its conjugated endings. Attaching the negative auxiliary verb ざる (zaru) required that す change to imperfective form, becoming せ (se). An alternate negative form using the same grammatical shift would be せず (sezu).
Anyway, the compound verb phrase above is marked as the topic of discussion by the particle は (wa) – if it makes you happy, imagine an implied の (no) or こと (koto) to nominalize it. Topic established, we comment on it. 勇 (yuu) is the noun “courage”; it is modified by the adjective 無し (nashi) in prenominal form. But the word that follows is simply the copula なり (nari), equivalent to modern である (de aru).
The whole thing, translated roughly and directly, thus becomes “As for seeing-and-not-doing morality, it is no courage.”
This kotowaza descends from a saying by Confucius in the Analects. 義 is one of the five Confucian virtues: in Japanese, 仁義礼智信 (Jin Gi Rei Chi Shin), “benevolence, justice, courtesy, wisdom, and sincerity.” This particular saying is attested at least as far back as the 14th Century.
Naturally, there’s no problem with writing 無き in hiragana only (なき). Some sources apparently read 勇 as isami.
「おまえ、なんでそんな厄介な事件に関わったんだい？」 「義を見てせざるは．．．」 「勇気なんて言うな、このアホ！死にたいのかよ？」
(“Omae, nande sonna yakkai na jiken ni kakawattan dai?” “Gi wo mite sezaru wa…” “Yuuki nante iu na, kono aho! Shinitai no ka yo?”)
[“Why the hell did you get caught up in something like that?” “’To see and not do…‘” “Don’t talk to me about courage, idiot! Do you want to get yourself killed?”]