be talking through (one's) hat

1. To be saying foolish things; to be talking nonsense. Don't pay Jonathan any mind, Mary, he's just talking through his hat again. What's up with that weird guy on the corner? He's always talking through his hat to passersby. Please, she was talking through her hat for that entire speech. There's no way she'll be able to get property taxes around here lowered.

2. To be exaggerating one's achievements or knowledge of some subject; to be bluffing or boasting. 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 and law. There's no way you can beat me at this game, twerp. You're talking through your hat.

be talking through your hat

INFORMAL

If someone is talking through their hat, what they are saying is silly or totally incorrect. Mrs Smith had told Adam he was talking through his hat if he thought communism would work. Quite honestly, I think he was talking through his hat.