Jiao qian yan shen (交浅言深). The acquaintance is shallow, but the words are deep. The Handian dictionary defines it as talking about intimate matters with someone one does not know well, and says that it means speaking without proper measure. Confucius made a related point: “If a person can be spoken with and one does not speak, one loses the person. If a person cannot be spoken with and one speaks, one loses the words.” (Analects, chapter Wei Linggong)
The idiom comes from the Strategies of the Warring States, where the line reads: “Deep words in a shallow acquaintance are disorder.” Handian also cites a version of the same passage in which deep words in a shallow acquaintance are called loyalty.