。
I hear you've been up to some pretty nasty stuff.
Literal
Fairly nasty things [obj-を] done-[progressive-てきた] [hearsay-そうだな].
あくどい 'nasty / unscrupulous / garish' is a colloquial adjective. ~してきた uses the directional てくる to mark a history of actions leading up to now. ~そうだな is masculine hearsay — 'I hear that / so it seems' + な (masculine confirmation). The sentence has the tone of a confrontation or interrogation.