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.
Let me call up my bratishka, he'll hook us up with some deals.
Bratish', why you mad?