Gigantopotamus
1. A humorous name, term, or term of endearment for a fat, or slightly overweight, person.

2. A humorous name, or term, for a morbidly obese person, provided they don't hear you.

Woman: "Does my bum look big in this?"
Partner: "Of course it does Gigantopotamus"

From the words gigantic and hippopotamus.
See: Chunkosaurus