1) Не унывай : Cheer up!
Where does that come from?
The verb "унывать" derived from "уныние" (dejection). So the verb means "despond" and "be depressed". "Не унывай" means literally "don't be depressed", "don't fall into dejection".

Difference between око and глаз ?
"Око" (plural form: "очи") is outdated word. It's not using in every day speach, it's using only in literature, in high style, cause this word sounds impressively, wisely and even epic for modern russian ears. For example, "Eye of Sauron" is "Око Саурона" in Russian, not "Глаз Саурона".

Наряд
It derived from "рядиться/наряжаться" (dress up), so "нарядная одежда" or "наряд" means "dressy/fancy clothes". However I think "bright clothes" is more close translation, cause you can't say "нарядно одетая" about a woman in evening dress. In this case "нарядно одетая женщина" sounds bit guilelessly. Usually "нарядный" is used to describe a christmass tree, children clothes and folk or fete costumes.

what's the name for a specific place to buy meat (Boucherie in French)
скотобойня?
Мясная лавка.

The word "скот-о-бойня" derived from "скот" (livestock) and "бой" (fight) wich generates verbs "бить" (beat/strike), "убить" (had killed), "убивать" (to kill). So "скотобойня" means literally "a place were men kills livestock for meat" (slaughterhouse). Yes, you can buy fresh meat in such place too.

хищница
It's female form of the word "хищник" (predator).

Is this word [хищница] slang? You know for those dressed up girls?
No. It's not slang. In figurative sense it mens "aggressive" sexy girl.