keep (one) at arm's length

To keep one a certain distance away, either physical or figurative, in order to afford a level of safety or security or to prevent intimacy. John always keeps his friends at arm's length, so that no one gets close enough to hurt his feelings. My daughter likes to watch me cook, but I make sure to keep her at arm's length so she doesn't get close to anything hot.

keep someone at arm's length

If you keep someone at arm's length, you avoid being friendly with them or getting emotionally involved with them. Brian felt more guilt than grief. He'd tried to get close, but his father had kept him at arm's length.