Hold out on
Definition 1 of 2
Pro Tip 1/3Secretive Implications
Suggests the person is being intentionally secretive for personal reasons. 
He seems to be holding out on the details of his new project.
Pro Tip 2/3Trust Issues
May imply a lack of trust or fear of being judged. 
She's holding out on me; maybe she's afraid I'll react poorly.
Pro Tip 3/3Subtle Pressure
Often used to gently accuse someone of not being fully open. 
You're not holding out on me, are you? I thought we had no secrets.