Literally Bratishka means "little brother" in Russian (Brat = brother) .
Bratish' (brah-tEEsh) is a short form of "bratishka".
You can refer to your younger bros as bratishka(singular) or bratishki(plural). The term is mostly applied to those younger than you; sometimes it can be applied to bros of your age, and it's rare to heard it said to anyone older.
The word is widely used in post-Soviet countries among Russian and non-Russian speakers.
homie, bro, younger brother, good friend.
Bratishka! lemme get some cash, man.
Let me call up my bratishka, he'll hook us up with some deals.
Bratish', why you mad?