Like Senioritis, only worse. Strikes usually half way through the summer after coming home from college to live with your folks for four tortuous months. Symptoms include wearing headphones to block out excessive parental noise, spending your days 24/7 at work or at a friend's house, suddenly becoming introverted, or living entirely in your bedroom, just to escape from the phenomenon of parents wishing to speak to you about the most random shit.
Mom: "Honey, can you mow the lawn?"
Father: "I think he's got parenitis."