口が滑る (kuchi ga suberu)

Literally: The mouth slips

Example: “つい口が滑って、サプライズパーティーのことを話してしまった。” — I let the cat out of the bag about the surprise party without thinking.

English equivalent: To let the cat out of the bag

Origin: "Kuchi" (mouth) here stands for speech itself, and "suberu" (to slip) describes words escaping before the speaker can hold them back, the same way a foot slips on ice with no time to catch itself. One plausible account of its development points to an older, more neutral sense: a mouth or tongue that "slipped" easily was originally just a fluent, quick-talking one, and a habitually fluent talker is naturally more likely to occasionally blurt out something better left unsaid — a fairly small step from praising someone's fast speech to describing the specific accidental slip that fast speech makes more likely. A documented literary example appears in the 1801 haikai collection Haikai Bukuro, already using the image of a slipping mouth in connection with an old, half-forgotten romantic memory.