Dear Mav: I liked the last four lines of that link:
II. Extended o-grade form *poid-. fat, from Old English ft(t), fat, from Germanic past participle *faitidaz, fattened, from derivative verb *faitjan, to fatten, from *faitaz, plump, fat. (Pokorny pe()- 793.)

"faitaz" = callipygy