“信口开河”，汉语成语，字面意思“speak carelessly, rapidly, voluminously like the outflow of river water when the sluice gates are opened”。比喻随口乱说一气，指说话没有根据，不可靠（speak without measuring one's words）。可以翻译为“talk through one's hat”，表示“talk nonsense”。
Don't pay Jonathan any mind. He's just talking through his hat again.
Don't talk through your hat!
He could not have seen me yesterday, because I wasn't there.