Tā xìnkǒu shuō le yíjùhuà, méixiǎngdào yǐnqǐ le wùhuì.
He blurted out a remark without thinking and unexpectedly caused a misunderstanding.
Tips
usage
信口 is often followed by verbs like 说 (say), 答应 (promise), or appears in the fixed phrase 信口开河 (to talk carelessly / to speak off the cuff). It carries a mildly negative connotation of irresponsibility.