Fatmath is the mental calculation one must do of the relative obesity of everyone in the room before you mention (even in passing) your weight loss plans or act disgusted with your recent weight gain.
Failure to do fatmath can result in fielding a hearty stab to the face from someone greater than or equal to your degree of fattitude, even though the food didn't jump down your goddamn throats, people.
Rosencrantz: We were all eating birthday cake, and I said "Man, I must have gained 5 pounds this summer, I need to go on a diet" and then some 180 pound woman comes out of nowhere and stabs me directly in the face!
Guildenstern: Dude, clearly you did not do the fatmath
in this situation.