I was ready to be mad at you for making me google it, but it turned out to be the same iusnaturalist bullcrap that was already centuries out of date when I studied that stuff and had memory holed, so... meh.
Fond memories of my college years, though. Feeling young and smart and so, so intellectually superior by pointing and laughing at those guys because back then we all thought things were mostly going to get better looking forward. Good times.
After a bit of research, my top guess is Body Size Dissatisfaction (I.E., wanting to lose weight).
But it could also be Bermuda Shorts Day or Bungo Stray Dogs.