He's talking through his hat.
【讲解】To talk through one's hat这句俗语是指说话的人对主题根本不懂，胡说八道。
Anybody who says we can balance the budget without raising taxes is just talking through his hat.
Hey, don't listen to that guy. He's talking through his hat when he tells people that two-headed men from Mars have landed a spaceship near Washington and the government is keeping it secret.