Well; it's a new hip word with the youngens these days. It just means "that's how i feel right now" or "extremely relatable". They use it primarily when that is the heaviest emotion in their life at that point in time.
The mood plays the most important role in decision making. You could blame the mood when you don’t want to do something. You must always consider the mood.
Timmy: “HeyBilly! Tryna go play ball today?”
Billy “ What about the mood? You know, I’m just not really in the mood.”
A fat mood is just one of those moods where you want to pull up at McDonald’s and order like 50 chicken nuggets bc a fat mood it’s also something you say when something just sounds good or relatable. The best way to say ‘a fat mood’ is to shout it very loud and annoyingly
Wow I just ate some good ass chicken nuggets what a fat mood!