On the surface, of course, Nostromo's "undertaking" refers to the removal of the silver, and we later learn that Monygham does not approve of the plan , hence his sour nod. But the line might have an allegorical double meaning, in which Nostromo's "undertaking" stands for the role of the People in total, a role that Cynicism looks "sourly" upon because it makes possible the futile and tragic dream-ideals of history, and because of the People's naiive faith in betterment. See where Monygham calls Nostromo "a fool" for that faith.