Secretive (adjective)

1. Tending to keep secrets or information to oneself.
2. Reserved in speech; unwilling to share information.

Origin:

From "secret".

Examples:

1. He was secretive about his plans for the future.
2. She was secretive about her personal life.
3. He had a secretive look on his face when I asked him about it.
4. She always seemed so secretive, I never knew what she was thinking.
5. He was very secretive about his business dealings.