being as a fat person would be, and sometimes being an all around loser. other times it can be used as a reason on why something is wrong.
eating like you have never ate before, doing things that are stupid that nobody think is funny or can be used like this: "well the car doesnt work because its being fat" or "no you cant do (insert something here) because you are being fat"
Sitting around eating, watching tv, playing video games, messing around on the computer, and/or listening to music (if you aren't going to dance or move much).
"What'd you do all day?" "Oh I was just being fat."
"Did you start your essay yet?" "No, I'm being fat."