口くちの堅かたい

expression, i-adjective

tight-lipped; able to keep a secret

Describes a person who does not reveal secrets or private matters. Often used in the negative form 口の堅くない to mean someone who cannot keep a secret.

See also: 口が堅い

• 彼かれは口くちの堅かたい人ひとだから、秘ひ密みつを話はなしても大だい丈じょう夫ぶだ。

  He's a tight-lipped person, so it's safe to tell him a secret.

• 口くちの堅かたい同どう僚りょうにだけ、その情じょう報ほうを伝つたえた。

  I only shared that information with a colleague who can keep a secret.