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1.
(n) a day where you feel fat. its not your period...its not that mcflurry and vodka you mixed together last night, its just a fat...day..

-you feel like an oompah loompah trapped inside of the Pepperidge Farm. but its okay, cuz everyone has them.

-just grab a pair of sweats and one of those t-shirts you

you got for free at your soccer camp...and lets rock

these rolls, baby!
Amelita "eww..whats she wearing?"

Carlos "her sweats and a soccer shirt!"
Amelita "ohhh...."
(together) "FAT DAY!!! WOOOHOOO!!!"
by loompadydoompah March 08, 2010
 
2.
A day of the week/month in which you deside you will eat whatever you want, usually including your friends in it.
When we get to the cinema, we're having a fat day! We'll buy huge popcorn packs and tons of jellies!
by NightOwl5 January 19, 2014
 
3.
1. The act of taking a random day off from school and using your weight and/or size as your excuse.
2. The act of taking a day off of gym class due to the activity or just pure laziness due to your morbid obesity.
Hey man, we're running in gym today. I think I'll take a fat day.
by x Ant e Dote November 14, 2003