Meatrock means: A piece of highly processed mechanically separated chicken that is used as an ingredient for meals served in the US prison system.. It is informal slang, fine in casual speech but not suited to formal contexts. It comes from prison or criminal-justice contexts; worth knowing what media your teen is consuming. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.