talk through (one's) hat
1. To say foolish things; to talk nonsense. Don't pay Jonathan any mind, Mary, he's just talking through his hat again.
2. To exaggerate one's achievements or knowledge; to bluff or boast. Dorothy keeps saying she can outrun anyone in our school, but she's talking through her hat if you ask me. It seemed like the candidate was talking through his hat for a while when the debate turned to the topic of tax policy.