口が滑る。

The idiom '口が滑る' (kuchi ga suberu) directly translates to 'the mouth slips.' Its origin can be understood by examining the individual components and the metaphorical sense they convey. '口' (kuchi) means 'mouth.' In Japanese, as in many languages, the mouth is directly associated with speech and communication. '滑る' (suberu) means 'to slip,' 'to slide,' or 'to glide.' This verb is often used to describe something moving unintentionally or losing its grip. For example, '手が滑る' (te ga suberu) means 'one's hand slips,' referring to accidentally dropping something. When these two words are combined, '口が滑る' creates a vivid image of words or secrets unintentionally slipping out of one's mouth, much like an object slipping from one's hand. The implication is that the speaker did not intend to reveal the information, and the utterance was an accident or a moment of carelessness. This idiom likely developed from the common human experience of accidentally blurting out something one meant to keep secret. The imagery of 'slipping' effectively captures the involuntary and sometimes regrettable nature of such an utterance. It emphasizes the lack of control the speaker had over their words in that particular moment. While it's difficult to pinpoint an exact historical document or period for its first use, the construction of this idiom is consistent with many other Japanese metaphorical expressions that use physical actions to describe internal or social phenomena. The simplicity and directness of the image suggest it has been in use for a considerable time, resonating with the universal experience of verbal gaffes.