It is a term of endearment. In addition to meaning friend, Young Stubbs is also acknowledging that you "know what is up" and that you "got your smoking game on fleek" by using this old world slang.
Godsdamn team Percival sounds pretty crazy right now. I assume they have some role copiers to take Aldo's place even if they don't revive him. So weird for them to kill him but it might have made sense to them? I can't really condemn based on 3rd hand info.