I tell ya to do a thing, you fucking do it, you got that?
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French: In the future, I tell ya to do a thing, you fucking do it, you got that?
Costigan: I got it, okay?
Frank: Excuse me, uh, French, I forgot my...
French: Okay, Francis
Frank: Cigarette. Eat something.

French is telling Costigan that in the future if he tells Costigan to do something then he had better do it.