信口开合 Talk casually
Explanation
形容说话随便，不加考虑，乱说一气。
Describes someone who speaks casually, without thinking, and says whatever comes to mind.
Origin Story
从前，有个读书人名叫张三，为人夸夸其谈，喜欢信口开合。一天，他去参加县令的宴会，席间，他为了显摆自己的学识，信口开河，大谈诗词歌赋，甚至还编造了一些虚假的故事。县令是个精明人，一眼就看穿了他的把戏，脸上虽然不动声色，心里却暗暗好笑。宴席结束后，县令并没有责怪张三，只是意味深长地对他说：『说话要谨慎，切不可信口开合，否则会给自己带来麻烦。』张三听了县令的话，羞愧难当，从此以后，他改掉了信口开合的坏毛病，为人也变得更加稳重了。
Once upon a time, there was a scholar named Zhang San, known for his boastful talk and tendency to speak without thinking. One day, he attended a banquet hosted by the county magistrate. To show off his knowledge, he talked endlessly about poetry, songs, and prose, even fabricating false stories. The magistrate, a shrewd man, immediately saw through his trick. Though he remained impassive, he secretly amused himself. After the banquet, the magistrate did not reprimand Zhang San, but meaningfully said to him, 'Be cautious with your words, and don't speak without thinking; otherwise, you may bring trouble upon yourself.' Zhang San, ashamed, corrected his bad habit of speaking thoughtlessly and became more composed.
Usage
多用于批评或讽刺别人说话不经大脑，随便乱说。
Often used to criticize or satirize someone who speaks thoughtlessly and randomly.
Examples
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他信口开河，说了许多不着边际的话。
ta xinkou kaihe, shuole xudu bu zhaobianji dehua.
He spoke without thinking and said many irrelevant things.
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会议上，他信口开合，差点惹出大乱。
huiyi shang, ta xinkou kaihe, chadian rechu daluan.
At the meeting, he talked nonsense and almost caused a big commotion.