Antonyms for the phrase "putting in plain English" could be using jargon, confusing, obfuscating, or complicating. When someone is not putting something in plain English, they might be using technical language or terminology that is not familiar to the listener or reader. This can make it difficult for the audience to understand the information being presented. When information is confusing or obfuscated, it can be frustrating for the person trying to comprehend it. On the other hand, when information is presented in plain English, people can easily comprehend it, and it can create effective communication between individuals.