Talk through your hat

Talk through your hat: falar sobre algo que não se conhece bem, afirmar que algo é verdade quando não é

“Evan says that quantum physics proves that space and time don't really exist, but Kathy says he's talking through his hat.”

“I proved that George was talking through his hat by checking on the internet to find out if what he said is true - and it isn't.” (www.englishclub.com)