It means "fat inspiration" but it's a very stupid word. It's the opposite of "thinspo" which is the shortened version of "thinspiration".  Thinspiration, or "thinspo" for short, is a clever amalgamation of the words thin and inspiration that makes perfect sense.  Thin people post pictures of themselves with the tags "thinspiration" or "thinspo" as inspiration for others to be thin too. "Fatspo" was created as the opposite, which means that in the long form it's "fatspiration". It's an idiotic word that makes no sense at all unless you're aware of the opposite, i.e. "thinspo". Basically fat people got upset by healthy or skinny people posting pictures with the tag "thinspo"/thinspiration, trying to inspire others to get healthy too. So they started posting pictures of themselves with a "fatspo" tag on it to encourage others to be fat instead, even though "fatspo" makes no sense.