口くちを滑すべらす

expression, godan verb (-su)

make a slip of the tongue; blurt out; say inadvertently

Used when someone accidentally says something they shouldn't, often revealing a secret or true feeling.

• つい口くちを滑すべらせて、秘ひ密みつを話はなしてしまった。

  I accidentally let the secret slip out.

• 彼かれは酒さけに酔よって口くちを滑すべらせた。

  He got drunk and blurted it out.