Fattitude is a word people use to talk about how they feel about their bodies when they are bigger than what other people think they "should" be. It means feeling good about yourself and not letting other people's opinions about your body make you feel bad.
Imagine you have a friend who is really good at basketball. They love playing and they're really good at it. But then someone else comes along and tells them that they aren't good at basketball and they should stop playing. Your friend might start to feel bad about themselves even though they were having fun before.
It's kind of like that with your body. Sometimes other people might tell you that you're too big or you shouldn't eat certain foods. But if you have fattitude, you know that your body is just fine the way it is and you don't have to listen to what other people say. You can feel good about yourself and be proud of who you are, no matter what your body looks like.