Tight-lipped (adjective)

Not willing to talk or reveal information.

Origin:

Early 20th century: from the phrase tight-lipped, meaning having the lips pressed firmly together, suggesting reticence or secrecy.

Examples:

1. He was tight-lipped about his plans for the future.
2. She was tight-lipped about the details of the project.
3. He was tight-lipped about his personal life.
4. The company was tight-lipped about the upcoming changes.
5. The witness was tight-lipped about what she saw.