look up any word, like kappa:
 
1.
an intellectual state in which one is of well below average intelligence.

100% of the time, the mentally fat person is completely unaware of their intellectual condition.

Note: Physically attractive people can most certainly (and often times are) mentally fat.
Man, she was so hot but seriously, that girl is so goddamn mentally fat I could not stand another second in her presence.
by Brian Adelman May 01, 2010
 
2.
A term that can be used to describe someone who is lazy, or even just stingy. A person who hogs all of their food, even if a starving person asks politely.
Wyatt: "I didn't have any breakfast, and Spencer is being Mentally Fat."
Spencer: "I'm not fat!"
Wyatt: "I said you were Mentally Fat!"
by Adumb50 May 13, 2010
 
3.
not physically being fat, but knowing that you would be fat because all you do is gobble down food.
Tina was mentally fat after her third plate of nachos.
by SARIE777777 March 12, 2008