It is an adverbial phrase mode, or purpose, for its definitive sense. It is said of some action, response or manifestation of someone to another person is now over who the patience and forgiveness toward the first. So that, when someone gets tired of supporting non-attitudes, metaphorically, is taken one as the definitive one, using the metaphor of the glass to overflow and the action as that " drop " that breaks the edge, the limit of patience.
