It's a cat that has gotten fat enough over the years to swallow a thermonuclear bomb and live, but heavy enough that its rolls of fat cause enough pressure to detonate the bomb.
EDIT: I googled it and apparently it's from this game "Dwarf Fortress", when cats and kittens are modded to have a body temperature of ~30000 C and put in a tight group they will "explode" and fuck everything over.
And strictly speaking, cats still create a catsplosion when unmodded due to their ridiculously high breeding rate, but a thermonuclear catsplosion just results in a faster growing, hotter wasteland of cat parts. But, I don't play Dwarf Fortress that much...