keep someone or something at arm's length
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.