Slank is a general term similar to dank in that it can be used to describe something of quality or to refer to drugs, but with a more slimey, unsavoury connotation.
Give me some of dat slank bruh
That shit was slanky as hell
When a giy decides to treat himself with a slow wank. Slow + Wank = Slank.
Langers: What are you up to tonight?
Obars: Considering treating myself to a nice slank!