1. To speak nonsensically or foolishly. He's talking out of the back of his head again, saying that he saw a kangaroo driving a car.

2. To exaggerate one's achievements or knowledge of some subject; to bluff or boast. Dorothy keeps saying she can outrun anyone in our school, but she's talking out of the back of her head if you ask me. It seemed like the candidate was talking out of the back of his head for a while when the debate turned to the topic of tax policy and law.