따거 came from a Chinese word, 大哥. It's same with the Korean word meaning 'elder brother', 형님. 따거 is a honorable term for an older brother or a superior. You can use this even if you are not blood related with them if you are close enough to them. But it's not formal though. The same goes for 형님, or 'Aniki', the same version of Japanese. But I really never heard of that in real life. I just know this because I learnt Chinese in my high school as a second foreign language. FYI, '형님' is usually used between old people, even between women. But younger generations don't use it anymore in my opinion, because it sounds too honorific. Nowadays people normally ditch their honorifics when closed enough each other. So there is no point of going back in using honorifics and make things awkward you know.

in france, that means "okay" :D but im not sure about korean. Ddako , sounds like d'accord