A middle school jab goes something like this: "We're having an A-B conversation, so you can C your way out." I bring this up because there's a workplace parallel to this that doesn't seem to have a name. It's when you're having an A-B email conversation and one party suddenly copies your boss, manager or someone more senior, in order to get an advantage in the discussion at hand.
Let's call it the acc. The A can stand for angry, awkward or annoying. We already know blind copying (bcc) can be toxic (read: congressional aide Kurt Bardella's secretly copied emails to a New York Times reporter that led to his firing), and openly copying more interested parties in a benign situation usually doesn't bother anyone. But it is not cool to use the cc as a weapon. And that is the stuff of the acc.
Our stab at a working definition for the acc: The situation in which new recipients are unexpectedly added to an existing email chain by one of the original parties with the intent to undermine the other original party's position.
Putting aside that it's not fair to the third party who's getting dragged into a situation for which he lacks context, the acc is just an unnecessary, passive-aggressive move that blindsides the original party. The point you are making should either be valid on its own, or part of a wider conversation to begin with. There's an implicit rank-pulling or tattling involved when you add an acc; bringing in someone else is no way to e-behave.
As far as we can tell, this irritating practice doesn't have a widely adopted name. Don't know if acc will stick, but let's try it out, folks. And if you want to, email me about it. Just be mindful of whom you cc.