When one says/does something mean, that they would not normally do, just because they're angry. Generally unnecessarily mean in proportion to the level of anger. Often a symptom of being butthurt.
Girl 1: (joke)
Girl 2: Sounds like something my 7-year-old cousin would say.
Girl 1: Well, that sounds like something a 20-year-old with an unfortunate face would say. What is that, partial facial paralysis?
Girl 3: Dang, you get meangry quick. Girl 2 has a cute face.
To be so hungry you start gettin mean and angry.
I need to eat, I am geting Meangry.