Fat is an adjective, not an insult. Originally it was a word created to describe a person’s size. We’re reclaiming that meaning. Fat does not mean lazy, sloppy, clumsy, unworthy, etc. Fat is just the size of a person and fat people are diverse and beautiful.
If you’d like to learn more about the reclamation of the word fat here are some great articles on the subject:
'Fat Is Not A Bad Word' by Fatima Mohammed.
'Fat Is Not A Bad Word' by Ashleigh Shackelford.