When a person acts in a passive-aggressive manner, they are displaying aggression in a way that is indirect as opposed to direct (like hitting or yelling). There is no direct anger or confrontation involved, but the person is expressing aggression indirectly. For example, if you are angry at your spouse who asked you to pick up several ingredients for dinner that night, and you somehow forget a couple of the items which make preparing the meal impossible, this might be considered a passive aggressive act.