the end result, no matter how sexy ur tummy is beforehand, of eating excessive amounts of food usually at a family gathering of some sort and ending up with a little round belly. no worries, fat tummy disappears after waking from your slumber caused by the "itis". fat tummy is a "condition" rather than a full-blown noun
P1:"ohhh man look at how sexy and toned my stomach is"
*one family reunion later*
P1:"aaagh im soo fat oh no!"
P2:"dont worry thats just fat tummy"