expression, i-adjective
tight-lipped; able to keep a secret
Describes a person who does not reveal secrets or private matters. Often used positively to indicate trustworthiness.
See also: 口の堅い
Antonyms: 口が軽い
彼は口が堅いから、秘密を話しても大丈夫だ。
He is tight-lipped, so it's safe to tell him a secret.
口が堅い人にだけこの情報を伝えてください。
Please share this information only with people who can keep a secret.