Dead agenting is a term used when someone makes another person look bad or wrong by spreading lies or false information about them. It's like if you're playing with a toy and someone else wants to play with it, but they can't have it, so they say the toy is bad or broken to make you not want it anymore.
Sometimes people do this on purpose to make others feel bad or to make themselves look better. But it's not a nice thing to do, and it can hurt people's feelings. It's important to always tell the truth and treat others with kindness and respect.