"At arm's length" means that the two parties are impersonal business acquaintances. They treat each other just like they would treat anyone else with the same

"At arm's length" means that the two parties are impersonal business acquaintances. They treat each other just like they would treat anyone else with the same amount of goods to sell or money to spend. When you sell a car to your nephew for $1, that is not an "arm's length" trade. In some situations, a trade that is not "at arm's length" may be taxed on an independently assessed value, rather than the actual purchase price. In most places in the US, arm's length is literally the distance you stand from someone when closing a business deal. You can't shake hands from further away, and you can't hug each other without getting closer. OP might have meant something along the lines of "keep your friends close, and your enemies closer." Or you can't be heard if you aren't even there to speak. I'm not sure if there's another idiom that would be more precise without also being less concise, but "arm's length" is probably not the best phrasing.